Fix new-game title flip + cap lookahead prepare burst

Builds on the worker migration with prepare-burst pacing and a title-flip fix:

- New game from mid-game left the book on the previous game's spread, so the first block's
  source and target spread matched and the title->content page turn was skipped. story:client-reset
  now returns the book to the title spread (spread 0) so the first block flips 0->1 and animates.
  Verified: requiresSpreadTransition src=0 tgt=1, page-flip-started/near-end fire.

- The lookahead burst-prepared many blocks at once, spiking allocation/GC into multi-second
  main-thread stalls. WebGL book prepares are now serialized through a chain and capped to a
  small lookahead window (TTS audio prefetch still spans the full window); future lookahead is
  also deferred until the current sentence has entered the display pipeline, keeping it off the
  first flip/reveal critical path. Worst game-start stall ~6s -> ~3.4s.

- Page flips now drive the scene through the sceneControl prewarm/startPreparedPageFlip API
  (awaited) instead of an event, and the scene awaits the async initial spread draw.

Suite 177. Remaining: a per-block prepare stall (~1.6-3.4s for large blocks at game start)
that profiling has not yet attributed to a single function (likely GC from prepare-path
allocation) — needs a DevTools performance capture for exact attribution.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/check-webgl-book-lab.js

@@ -129,6 +129,7 @@ const checks = [
     ['webgl lab exposes reveal uniform diagnostics', /getRevealDebugState/.test(source) && /bookRevealActive/.test(source) && /bookRevealElapsedMs/.test(source) && /bookRevealRegionCount/.test(source)],
     ['webgl lab records page reveal clear reasons', /clearPageReveal\(side, reason/.test(source) && /webglRevealClearLog/.test(source)],
     ['webgl reveal clock starts on first render frame', /pendingStart/.test(source) && /state\.pendingStart/.test(source) && /state\.startedAt = now/.test(source)],
+    ['webgl reveal start survives event-before-state ordering', /function getRevealStartTimeForBlockIds/.test(source) && /activeRevealBlockStarts\.set\(pendingBlockId, now\)/.test(source) && /pendingRevealStartBlockIds\.delete\(pendingBlockId\)/.test(source)],
     ['webgl reveal visual clock is derived from absolute playback time', /visualElapsedMs/.test(source) && /activeRevealBlockStarts/.test(source) && /state\.visualElapsedMs = Math\.max\(0, now - state\.startedAt\)/.test(source) && !/revealFrameDeltaMs/.test(source)],
     ['webgl fast-forward accelerates reveal instead of clearing the mask immediately', /fastForwarding/.test(source) && /fastForwardDurationMs/.test(source) && !/clearPageReveal\(side, 'fast-forward'\)/.test(source)],
     ['webgl lab records page texture binding timings', /pageTextureTimings/.test(source) && /markPageTextureTiming/.test(source) && /webglPageTextureTimings/.test(source)],
@@ -139,10 +140,13 @@ const checks = [
     ['texture renderer diagnostics include reveal region counts', /regionCounts/.test(textureRendererSource) && /lineRects/.test(textureRendererSource) && /durationMs/.test(textureRendererSource)],
     ['texture renderer does not draw immediate non-pending sides during pending reveal preparation', !/immediateSides/.test(textureRendererSource) && !/this\.drawSpread\(this\.currentSpread, immediateSides\)/.test(textureRendererSource)],
     ['sentence queue consumes completed prepared lookahead items', /preparedSentenceCache/.test(sentenceQueueSource) && /this\.preparedSentenceCache\.get\(cacheKey\)/.test(sentenceQueueSource) && /return prefetched/.test(sentenceQueueSource)],
-    ['sentence queue front-loads 3D book presentation before playback callback', /const sentence = await this\.getPreparedSentence\(item\);[\s\S]*await this\.prefetchWebGLBookPresentation\(sentence[\s\S]*immediate: true[\s\S]*this\.prefetchAhead\(6, queueGeneration\);[\s\S]*this\.onSentenceReadyCallback/.test(sentenceQueueSource)],
+    ['sentence queue starts future lookahead only after current display playback is entered', /const sentence = await this\.getPreparedSentence\(item\);[\s\S]*await this\.prefetchWebGLBookPresentation\(sentence[\s\S]*immediate: true[\s\S]*const playbackFinished = new Promise/.test(sentenceQueueSource) && /this\.onSentenceReadyCallback\(sentence, resolve\);[\s\S]*window\.requestAnimationFrame\(\(\) => \{[\s\S]*this\.prefetchAhead\(6, queueGeneration\);[\s\S]*await playbackFinished/.test(sentenceQueueSource)],
     ['sentence queue prefetch prepares whole future sentence instead of speech metadata only', /this\.prepareSentence\(nextItem, \{\s*blocking: false/.test(sentenceQueueSource) && /this\.prefetchWebGLBookPresentation\(prepared/.test(sentenceQueueSource)],
     ['sentence queue starts lookahead when items arrive during playback', /else \{\s*this\.prefetchAhead\(6, this\.queueGeneration\);/.test(sentenceQueueSource)],
     ['sentence queue keeps current 3D page prep immediate while future lookahead yields cooperatively', /if \(!options\.immediate\) \{[\s\S]*requestIdleCallback[\s\S]*timeout: 80/.test(sentenceQueueSource) && /prefetchAhead\(maxLookahead = 6/.test(sentenceQueueSource)],
+    ['sentence queue serializes heavy WebGL book preparation separately from speech prefetch', /prefetchingWebGLBook = new Map/.test(sentenceQueueSource) && /webglBookPrepareChain = Promise\.resolve\(\)/.test(sentenceQueueSource) && /this\.webglBookPrepareChain[\s\S]*\.then\(\(\) => this\.runWebGLBookPresentationPrepare/.test(sentenceQueueSource)],
+    ['sentence queue caps WebGL book lookahead without capping TTS lookahead window', /const WEBGL_BOOK_PREFETCH_LOOKAHEAD = 2/.test(sentenceQueueSource) && /webglBookLookahead < WEBGL_BOOK_PREFETCH_LOOKAHEAD/.test(sentenceQueueSource) && !/spokenPrepared >= 1 && started >= 2/.test(sentenceQueueSource)],
+    ['sentence queue gates WebGL book lookahead to active 3D playback only', /const allowWebGLBookPrefetch = document\.documentElement\.dataset\.webglBookPlaybackActive === 'true'/.test(sentenceQueueSource) && /const shouldPrepareWebGLBook = allowWebGLBookPrefetch[\s\S]*&& webglBookCandidate[\s\S]*&& webglBookLookahead < WEBGL_BOOK_PREFETCH_LOOKAHEAD/.test(sentenceQueueSource)],
     ['pagination can prepare future spreads without activating visible spread', /preparePendingBlock\(block = \{\}, options = \{\}\)/.test(bookPaginationSource) && /options\.activate !== false/.test(bookPaginationSource) && /includeUnrenderedHistory/.test(bookPaginationSource)],
     ['pagination preserves active inline style tags for texture lines', /getActiveStyleTags/.test(bookPaginationSource) && /activeStyleTags/.test(bookPaginationSource) && /updateStyleTagStack/.test(bookPaginationSource)],
     ['texture renderer stores prepared reveal plans in the shared texture store', !/preparedRevealCache/.test(textureRendererSource) && /rememberPreparedRevealPlan/.test(webglPageCacheSource) && /takePreparedRevealPlan/.test(textureRendererSource) && /publishPreparedReveal/.test(textureRendererSource) && !/hasPreparedRevealBlock/.test(textureRendererSource)],
@@ -161,7 +165,7 @@ const checks = [
     ['webgl debug test hook awaits the same async page flip path', /startPageFlipForTest\(direction, options = \{\}\) \{[\s\S]*return startPageFlip\(direction, options\)/.test(source)],
     ['webgl debug test hook can deterministically finish an active page flip', /advancePageFlipForTest\(elapsedMs = normalFlipDuration \+ 16\)/.test(source) && /updateActiveFlips\(targetNow\)/.test(source)],
     ['sentence queue skips duplicate current-item 3D book presentation when reveal is cached', /isWebGLBookPresentationPrepared/.test(sentenceQueueSource) && /if \(!this\.isWebGLBookPresentationPrepared\(sentence\)\) \{\s*await this\.prefetchWebGLBookPresentation/.test(sentenceQueueSource) && /sentence\.webglBookPresentation = \{\s*prepared: true/.test(sentenceQueueSource)],
-    ['3D overflow reveal commits the spread then requests a timeline flip via event before activating', /requiresSpreadTransition\(segment\)/.test(bookPlaybackTimelineSource) && /this\.commitSegmentSpread\(segment, sentence\)/.test(bookPlaybackTimelineSource) && /this\.requestPageFlip\(1, \{[\s\S]*targetSpread: segment\.targetSpreadIndex/.test(bookPlaybackTimelineSource) && /this\.activatePreparedSegment\(segment, sentence\)/.test(bookPlaybackTimelineSource) && /dispatchEvent\(new CustomEvent\('webgl-book:request-page-flip'/.test(bookPlaybackTimelineSource) && /addEventListener\('webgl-book:request-page-flip'/.test(source) && /startPageFlip\(direction, \{/.test(source)],
+    ['3D overflow reveal commits the spread then starts a prepared timeline flip before activating', /requiresSpreadTransition\(segment\)/.test(bookPlaybackTimelineSource) && /this\.commitSegmentSpread\(segment, sentence\)/.test(bookPlaybackTimelineSource) && /this\.requestPageFlip\(1, \{[\s\S]*targetSpread: segment\.targetSpreadIndex/.test(bookPlaybackTimelineSource) && /this\.activatePreparedSegment\(segment, sentence\)/.test(bookPlaybackTimelineSource) && /sceneControl\.prewarmPageFlip/.test(bookPlaybackTimelineSource) && /sceneControl\.startPreparedPageFlip/.test(bookPlaybackTimelineSource) && !/dispatchEvent\(new CustomEvent\('webgl-book:request-page-flip'/.test(bookPlaybackTimelineSource) && /prewarmPageFlip: \(direction = 1, options = \{\}\)/.test(source) && /startPreparedPageFlip: \(direction = 1, options = \{\}\)/.test(source)],
     ['texture worker paints inline bold and italic styles off the main thread', /getInlineStyleState/.test(textureWorkerSource) && /updateInlineStyleState/.test(textureWorkerSource) && /getCanvasFont/.test(textureWorkerSource) && /segment\.style/.test(textureWorkerSource) && !/drawLine\(ctx/.test(textureRendererSource)],
     ['texture renderer delegates page rasterization to an OffscreenCanvas worker and blits the result', /book-texture-worker\.js/.test(textureRendererSource) && /rasterizeSpread/.test(textureRendererSource) && /ctx\.drawImage\(result\.pageBitmap, 0, 0\)/.test(textureRendererSource) && /OffscreenCanvas/.test(textureWorkerSource) && /createImageBitmap/.test(textureWorkerSource)],
     ['texture renderer recovers from worker error/timeout so a draw promise never hangs the chain', /this\.rasterWorker\.onerror/.test(textureRendererSource) && /texture-worker-timeout/.test(textureRendererSource) && /settleRasterization/.test(textureRendererSource) && /clearTimeout\(pending\.timer\)/.test(textureRendererSource)],
@@ -174,7 +178,7 @@ const checks = [
     ['webgl page format reduces only outer margins from previous value', /outerBaseIn: 0\.27/.test(pageFormatSource) && /outerThicknessFactor: 0\.015/.test(pageFormatSource) && /outerMaxIn: 0\.315/.test(pageFormatSource) && /innerBaseIn: 0\.42/.test(pageFormatSource)],
     ['webgl mode enlarges and inverts DOM overlay text without touching 2D mode', /body\.webgl-mode \{[\s\S]*font-size: 18px;/.test(styleSource) && /body\.webgl-mode \.choice-list \.choice-button/.test(styleSource) && /rgba\(246, 231, 201/.test(styleSource)],
     ['webgl choice overlay hides title clutter and prevents horizontal scrollbar', /body\.webgl-mode #page_left #game_title/.test(styleSource) && /body\.webgl-mode #page_left #start_prompt/.test(styleSource) && /overflow-x: hidden/.test(styleSource) && /book\.style\.width = 'min\(44rem/.test(webglSceneSource)],
-    ['3D live text bypasses #page_right DOM rendering and uses book texture reveal directly', /const useWebGLBookReveal = this\.isWebGLMode\(\) && \(sentence\.kind === 'paragraph' \|\| sentence\.kind === 'heading'\)/.test(uiDisplayHandlerSource) && /if \(useWebGLBookReveal\) \{[\s\S]*await this\.prepareWebGLBookReveal\(sentence\);[\s\S]*await this\.playbackCoordinator\.play\(sentence\);[\s\S]*this\.markBlockRendered\(sentence\.blockId\);[\s\S]*return null;/.test(uiDisplayHandlerSource)],
+    ['3D live text bypasses #page_right DOM rendering and uses the timeline-owned book reveal directly', /const useWebGLBookReveal = this\.isWebGLMode\(\) && \(sentence\.kind === 'paragraph' \|\| sentence\.kind === 'heading'\)/.test(uiDisplayHandlerSource) && /if \(useWebGLBookReveal\) \{[\s\S]*await this\.playWebGLBookSentence\(sentence\);[\s\S]*this\.markBlockRendered\(sentence\.blockId\);[\s\S]*return null;/.test(uiDisplayHandlerSource) && !/if \(useWebGLBookReveal\) \{[\s\S]*await this\.prepareWebGLBookReveal\(sentence\);[\s\S]*await this\.playbackCoordinator\.play\(sentence\);[\s\S]*return null;/.test(uiDisplayHandlerSource)],
     ['drop-cap remaining text does not reinsert discretionary hyphen markers', /extractRemainingLayoutText/.test(bookPaginationSource) && !bookPaginationSource.includes("fragments.push('|')")],
     ['drop-cap reservation keeps a normal text gap beside the initial', /measureDropCapReservation/.test(bookPaginationSource) && /measureNormalTextGap\(fontPx\)/.test(bookPaginationSource)],
     ['drop-cap reservation uses both ink bounds and font advance width', /const advanceWidth = metrics\.width \|\| 0/.test(bookPaginationSource) && /Math\.max\(inkRight, advanceWidth, lineHeightPx \* 1\.08\)/.test(bookPaginationSource)],
@@ -197,6 +201,7 @@ const checks = [
     ['webgl texture store resident cache reuses newest page version for older readiness requests', /isOlderPageMeta/.test(webglPageCacheSource) && /getResidentTextureForMeta/.test(webglPageCacheSource) && /if \(!resident\) return null/.test(webglPageCacheSource) && !/if \(!resident \|\| this\.isOlderPageMeta\(pageMeta, resident\.pageMeta\)\) return null/.test(webglPageCacheSource)],
     ['webgl flip page is a two-sided body using paper-thickness constants', /flipPageBackSurface/.test(source) && /flipPageEdge/.test(source) && /new THREE\.Mesh\(geometry, \[\s*materials\.flipPageSurface,\s*materials\.flipPageBackSurface,\s*materials\.flipPageEdge\s*\]\)/.test(source) && /PROCEDURAL_BOOK\.SHEET_THICKNESS_MODEL/.test(source) && /const topMaterialIndex = direction > 0 \? 1 : 0/.test(source) && /const bottomMaterialIndex = direction > 0 \? 0 : 1/.test(source) && /geometry\.addGroup\(0, topIndices\.length, topMaterialIndex\)/.test(source) && /geometry\.addGroup\(topIndices\.length, bottomIndices\.length, bottomMaterialIndex\)/.test(source)],
     ['webgl animated page front and back maps are independently switchable before animation starts', /materials\.flipPageBackSurface = materials\.flipPageSurface\.clone\(\)/.test(source) && /materials\.flipPageSurface\.map = sourceTexture/.test(source) && /materials\.flipPageBackSurface\.map = backDeferred \? getBlankPageTexture\(\) : \(backTexture \|\| getBlankPageTexture\(\)\)/.test(source) && /flip\.sourceTexture = sourceTexture/.test(source) && /flip\.backTexture = backTexture \|\| getBlankPageTexture\(\)/.test(source)],
+    ['webgl flip page material variants are compiled during loader, not at first texture swap', /flipPageSurface: new THREE\.MeshStandardMaterial\(\{[\s\S]*map: getBlankPageTexture\(\),[\s\S]*normalMap: paperTextures\.normal,[\s\S]*roughnessMap: paperTextures\.roughness/.test(source) && !/materials\.flipPageSurface\.needsUpdate = true/.test(methodBody(source, 'prepareStaticPageForFlip')) && !/materials\.flipPageBackSurface\.needsUpdate = true/.test(methodBody(source, 'prepareStaticPageForFlip'))],
     ['webgl flip preflight exposes texture side and orientation invariants for browser tests', /lastFlipTexturePreflight/.test(source) && /sourceTextureMatchesBackTexture/.test(source) && /targetBackSide/.test(source) && /getRuntimeInvariants/.test(source)],
     ['webgl animated page maps source and destination textures to direction-aware physical sides', /const topPageSide = direction > 0 \? targetSide : sourceSide/.test(source) && /const bottomPageSide = direction > 0 \? sourceSide : targetSide/.test(source) && /topRow\.push\(push\(point, pageThickness, pageUvForSide\(topPageSide, u, v\)\)\)/.test(source) && /bottomRow\.push\(push\(point, 0, pageUvForSide\(bottomPageSide, u, v\)\)\)/.test(source) && /side < 0 \? 1 - pageU : pageU/.test(source) && /y: v/.test(source)],
     ['webgl animated page UVs use the same fore-edge inset as the visible stack cap', /PAGE_TEXTURE_FORE_EDGE_INSET_RATIO/.test(source) && /const pageU = THREE\.MathUtils\.clamp\(u \/ Math\.max\(0\.0001, 1 - inset\), 0, 1\)/.test(source)],
@@ -204,6 +209,8 @@ const checks = [
     ['webgl flip prewarm prepares current and target spread texture records before cache lookup', /prepareSpreadTextureRecordsForFlip\(currentSpread\)/.test(source) && /prepareSpreadTextureRecordsForFlip\(nextSpread\)/.test(source) && /function prepareSpreadTextureRecordsForFlip/.test(source) && /spreadTextureRecordsReady\(spread\)/.test(source) && /window\.BookTextureRenderer\.drawSpread\(spread, \['left', 'right'\], \{[\s\S]*phase: 'prepare'/.test(source)],
     ['webgl animated page reuses geometry buffers during flips', /function updateFlippingPageGeometry/.test(source) && /position\.needsUpdate = true/.test(source) && /updateFlippingPageGeometry\(flip\.mesh\.geometry, surface\)/.test(source) && !/flip\.mesh\.geometry\.dispose\(\);\s*flip\.mesh\.geometry = geometry;/.test(methodBody(source, 'setActivePageGeometry'))],
     ['webgl scene targets 60fps with browser-frame scheduling and staggered live mirror refresh', /const targetFrameDurationMs = 1000 \/ 60/.test(source) && /const minRenderFrameIntervalMs = targetFrameDurationMs \* 0\.5/.test(source) && /requestAnimationFrame\(animate\)/.test(source) && /const refreshStaticSceneBuffers = staticSceneBuffersDirty \|\| activeFlips\.length > 0/.test(source) && /const dynamicBufferRefreshIntervalMs = 1000 \/ 30/.test(source) && /const flipDynamicBufferGraceMs = 180/.test(source) && /const bothHeavyPassesDue = shadowRefreshDue && reflectionRefreshDue/.test(source) && /const refreshReflectionThisFrame/.test(source) && /updateTableReflection\(\);/.test(source) && /mirrorRefreshesAtFps/.test(source) && !/setTimeout\(animate/.test(source)],
+    ['webgl reveal playback throttles dynamic buffers without freezing mirror permanently', /const revealGeometryBufferRefreshIntervalMs = 1000 \/ 4/.test(source) && /const revealAnimating = hasActivePageReveal\(\)/.test(source) && /revealAnimating[\s\S]*revealGeometryBufferRefreshIntervalMs/.test(source)],
+    ['webgl navigation texture prewarm yields until reveal and flip critical frames are clear', /function scheduleNavigationTextureWindowPrewarm/.test(source) && /requestIdleCallback/.test(source) && /activeFlips\.length > 0 \|\| hasActivePageReveal\(\)/.test(source) && /scheduleNavigationTextureWindowPrewarm\('page-texture-records'/.test(source)],
     ['texture renderer has no private reveal clock (scene render loop is the single clock)', !/this\.targetFrameDurationMs/.test(textureRendererSource) && !/tickAnimations/.test(textureRendererSource) && !/requestAnimationFrame/.test(textureRendererSource)],
     ['webgl scene lowers mirror target and caps table film maps to 2k', /const reflectionPixelRatio = 0\.72/.test(source) && /const tableReflectionBaseWidth = 1536/.test(source) && /const tableReflectionBaseHeight = 864/.test(source) && /tableDustTexture = loadUtilityTexture\('\/assets\/webgl\/table_dust_4k\.png', \{ maxSize: 2048 \}\)/.test(source) && /tableGreaseTexture = loadUtilityTexture\('\/assets\/webgl\/table_grease_4k\.png', \{ maxSize: 2048 \}\)/.test(source)],
     ['webgl debug exposes runtime invariants for visual regression tests', /getRuntimeInvariants\(\)/.test(source) && /residentPageTextureCount/.test(source) && /flipFrontBackShareMaterial/.test(source) && /mirrorRefreshesAtFps/.test(source) && /mirrorDefersDuringFlipStartMs/.test(source)],
@@ -224,7 +231,7 @@ const checks = [
     ['webgl autoplay flip source prefers currently revealing visible material over resident cache', /if \(revealStateMatchesPage\(side, pageMeta\)\) return material\?\.map \|\| null/.test(source) && /revealStateMatchesPage\(sourceSide, sourcePageMeta\) \? sourceSide : null/.test(source)],
     ['webgl flipping page materials mirror active reveal shader uniforms on both sides', /materials\.flipPageSurface\.userData\.bookPageReveal/.test(source) && /syncFlipRevealShaderFromSource/.test(source) && /bookRevealRegionRects/.test(source) && /materials\.flipPageSurface\.userData\.sourceRevealSide === side/.test(source) && /revealStateMatchesPage\(targetBackSide, targetBackPageMeta\) \? targetBackSide : null/.test(source)],
     ['webgl prepared texture records do not mutate the visible page metadata', /const incomingPageMeta = detail\.pageMeta/.test(source) && /if \(detail\.phase !== 'prepare' && detail\.pageMeta\) \{[\s\S]*currentPageMeta = incomingPageMeta/.test(source) && /pageMeta: effectivePageMeta/.test(source)],
-    ['webgl scene force-redraws current pagination spread for initial title upload', /const initialSpread = pagination\?\.getCurrentSpread\?\.\(\)/.test(webglSceneSource) && /window\.BookTextureRenderer\.drawSpread\(initialSpread, \['left', 'right'\], \{ force: true \}\)/.test(webglSceneSource) && /options\.force !== true && phase !== 'prepare'/.test(textureRendererSource)],
+    ['webgl scene awaits current pagination spread redraw during loader initial title upload', /const initialSpread = pagination\?\.getCurrentSpread\?\.\(\)/.test(webglSceneSource) && /await window\.BookTextureRenderer\.drawSpread\(initialSpread, \['left', 'right'\], \{ force: true \}\)/.test(webglSceneSource) && !/Date\.now\(\)/.test(webglSceneSource) && /options\.force !== true && phase !== 'prepare'/.test(textureRendererSource)],
     ['texture renderer marks committed reveal blocks complete so pauses cannot replay them', /webgl-book:reveal-committed/.test(textureRendererSource) && /completeRevealBlockIds/.test(textureRendererSource) && /this\.revealedBlockIds\.add\(id\)/.test(textureRendererSource)],
     ['webgl timeline recalculates placeholder zero-duration reveal timings from TTS duration', /existingTimings/.test(bookPlaybackTimelineSource) && /existingDuration/.test(bookPlaybackTimelineSource) && /ttsDuration/.test(bookPlaybackTimelineSource) && /existingTimings\.length > 0 && \(existingDuration > 0 \|\| ttsDuration <= 0\)/.test(bookPlaybackTimelineSource)],
     ['webgl playback coordinator trusts timeline-prepared reveal timings without recomputing', !/calculateWordTimings/.test(methodBody(playbackCoordinatorSource, 'scheduleWebGLReveal')) && /single owner of reveal timing/.test(playbackCoordinatorSource) && /sentence\.webglRevealController\(/.test(playbackCoordinatorSource)],
@@ -232,6 +239,8 @@ const checks = [
     ['texture renderer prepares a spanning block continuation spread in the background and reuses it (no synchronous redraw on the critical path)', /revealSpreadSourceOverride/.test(textureRendererSource) && /prepareContinuationRevealPlan/.test(textureRendererSource) && /takeContinuationRevealPlan/.test(textureRendererSource) && /`\$\{id\}:cont`/.test(textureRendererSource) && /prepareContinuationRevealPlan/.test(bookPlaybackTimelineSource) && /takeContinuationRevealPlan/.test(bookPlaybackTimelineSource) && /previewSpreads/.test(bookPaginationSource)],
     ['texture renderer preloads every spread touched by an active reveal block', /preloadAdditionalRevealSpreads/.test(textureRendererSource) && /spreadContainsBlock/.test(textureRendererSource) && /this\.drawSpread\(spread, \['left', 'right'\], \{ phase: 'prepare' \}\)/.test(textureRendererSource)],
     ['book playback timeline has one prepare path: spanning-aware plan reused at activate and continuation, no fallback', /revealSpreadSourceOverride: spanningPreview \? detail\.previewSpreads : null/.test(textureRendererSource) && /this\.revealSpreadSourceOverride = options\.revealSpreadSourceOverride/.test(textureRendererSource) && !/forceRebuild/.test(textureRendererSource) && !/forceRebuild/.test(bookPlaybackTimelineSource) && !/spanningPlanPrepared/.test(bookPlaybackTimelineSource) && /const texturePlan = this\.textureRenderer\.takeContinuationRevealPlan\(segment\.blockId, spread\.index\)/.test(bookPlaybackTimelineSource)],
+    ['book playback timeline reuses prepared activation texture plan on the critical path', /let texturePlan = segment\.preparedTexturePlan/.test(bookPlaybackTimelineSource) && /\{ \.\.\.segment\.preparedTexturePlan, phase: 'activate' \}/.test(bookPlaybackTimelineSource) && /takePreparedRevealPlan\(segment\.blockId\)/.test(bookPlaybackTimelineSource) && /if \(!texturePlan\) \{[\s\S]*prepareRevealBlock/.test(bookPlaybackTimelineSource)],
+    ['book playback timeline compares preplay flip against source spread captured before commit', /segment\.sourceSpreadIndex = this\.getVisibleSpreadIndex\(\)/.test(bookPlaybackTimelineSource) && /segment\.sourceSpreadIndex = Number\.isFinite/.test(bookPlaybackTimelineSource) && /const sourceSpread = Number\.isFinite/.test(bookPlaybackTimelineSource) && /targetSpreadIndex \|\| 0\)\) > sourceSpread/.test(bookPlaybackTimelineSource)],
     ['webgl visible spread is owned by scene flips, not pagination publishes', /spreadUpdate:state-only/.test(source) && /webglBookPlaybackActive/.test(source) && /spreadUpdate:jump/.test(source) && /window\.BookTextureRenderer\?\.drawSpread\?\.\(spread, \['left', 'right'\], \{ force: true \}\)/.test(source)],
     ['3D overflow reveal preloads target spread before forced page flip', /createRevealDetail\(sentence, previewSpread, 'prepare'\)/.test(bookPlaybackTimelineSource) && /this\.textureRenderer\.prepareRevealBlock\(\s*[\s\S]*revealDetail[\s\S]*phase: 'prepare'[\s\S]*publishEvent: false/.test(bookPlaybackTimelineSource) && /this\.prewarmSegmentTextures\(segment\)/.test(bookPlaybackTimelineSource) && /this\.assertSegmentReady\(segment, 'prepare'\)/.test(bookPlaybackTimelineSource) && /reason: 'timeline-preplay-spread-transition'/.test(bookPlaybackTimelineSource)],
     ['book playback timeline is loaded through module infrastructure', /book-playback-timeline-module\.js/.test(loaderSource) && /super\('book-playback-timeline'/.test(bookPlaybackTimelineSource) && /reportProgress\(100, 'Book playback timeline ready'\)/.test(bookPlaybackTimelineSource)],