Reuse spanning-aware prepared plan at activate (kill pre-playback pause)

A spanning block stalled ~2.1s before it began playing: activate ran prepareRevealBlock
with forceRebuild, synchronously redrawing the start spread (and preloading the
continuation spread) on the main thread, because the lookahead plan had been built with
right-only timing before pagination committed the overflow.

Build the start-spread plan spanning-aware during lookahead instead: when the preview
layout shows the block overflows, derive its timing across both preview spreads (via the
revealSpreadSourceOverride) and cache it. activate then reuses that plan — the same fast
cached-plan path non-spanning blocks already use — with no synchronous redraw. forceRebuild
is kept only as a fallback when a block spans but was not prepared spanning-aware (e.g. an
immediate prepare with no preview layout), and an evicted plan still rebuilds correctly
because pagination is committed by then.

Verified live: the spanning block's pre-playback gap dropped from ~2088ms to 139ms (equal
to non-spanning blocks), while the right line still reveals over its area share (~3.3s),
the continuation still animates from the start, and there are no fast-forwards or problems.

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

scripts/check-webgl-book-lab.js

@@ -186,7 +186,7 @@ const checks = [
     ['webgl flip assigns explicit source and back page textures before animation starts', /resolveCurrentFlipSourceTexture\(sourceSide\)/.test(source) && /const targetBackSide = flip\.direction > 0 \? 'left' : 'right'/.test(source) && /const targetBackPageMeta = getPaginationPageMeta\(targetBackPageIndex\) \|\| makeBlankPageMeta\(targetBackPageIndex\)/.test(source) && /materials\.flipPageSurface\.map = sourceTexture/.test(source) && /materials\.flipPageBackSurface\.map = backDeferred \? getBlankPageTexture\(\) : \(backTexture \|\| getBlankPageTexture\(\)\)/.test(source)],
     ['webgl flip never falls back to the opposite visible stack for target back texture', /function resolveFlipBackTexture\(pageMeta = null, prewarmedTexture = null\)/.test(source) && source.includes('return pageTextureStore?.getResidentTextureForMeta?.(pageMeta);') && !/oppositeMaterial\?\.map/.test(methodBody(source, 'prepareStaticPageForFlip'))],
     ['webgl page texture record metadata normalizes omitted or null sides into explicit blank pages', /function normalizePageMetaPair/.test(source) && /function makeBlankPageMeta/.test(source) && /applyExplicitBlankPageTexture/.test(source) && /normalizePageTextureRecordDetail/.test(source) && !/hasLeftMeta/.test(methodBody(source, 'handlePageTextureRecords'))],
-    ['texture renderer publishes both spread sides for reveal preparation', /const sides = \['left', 'right'\]/.test(textureRendererSource) && /const published = this\.drawSpread\(spread, sides/.test(textureRendererSource) && /pageMeta: prepared\.pageMeta \|\| \{\}/.test(textureRendererSource)],
+    ['texture renderer publishes both spread sides for reveal preparation', /const sides = \['left', 'right'\]/.test(textureRendererSource) && /published = this\.drawSpread\(spread, sides/.test(textureRendererSource) && /pageMeta: prepared\.pageMeta \|\| \{\}/.test(textureRendererSource)],
     ['texture renderer marks page canvases with content versions before cache writes', /pageContentVersions/.test(textureRendererSource) && /buildPublishPageMeta/.test(textureRendererSource) && /completenessScore: \(maxBlockId \* 1000\) \+ lineCount/.test(textureRendererSource)],
     ['texture store queues newer same-page cache writes instead of dropping them', /storePageCanvas/.test(webglPageCacheSource) && /isOlderPageMeta/.test(webglPageCacheSource) && /const previousWrite = pending\?\.promise \|\| Promise\.resolve\(\)/.test(webglPageCacheSource) && /pendingPageWrites\.set\(key, \{[\s\S]*pageMeta: \{ \.\.\.\(pageMeta \|\| \{\}\) \}/.test(webglPageCacheSource)],
     ['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)],
@@ -226,8 +226,9 @@ const checks = [
     ['texture renderer derives reveal regions from the just-drawn preview spread before committed pagination', /if \(this\.currentSpread\) sourceSpreads\.push\(this\.currentSpread\)/.test(textureRendererSource) && /paginationSpreads\.forEach/.test(textureRendererSource) && /Number\(spread\.index\) === Number\(this\.currentSpread\.index\)/.test(textureRendererSource)],
     ['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 reuses the spanning-aware prepared plan at activate instead of a synchronous forceRebuild', /spanningPlanPrepared/.test(bookPlaybackTimelineSource) && /forceRebuild = segment\.spansToNextSpread === true && segment\.spanningPlanPrepared !== true/.test(bookPlaybackTimelineSource) && /spanningTimingPrepared/.test(textureRendererSource) && /this\.revealSpreadSourceOverride = detail\.previewSpreads/.test(textureRendererSource)],
     ['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\(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)],
+    ['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)],
     ['book playback timeline initializes before sentence queue without a dependency cycle', /this\.dependencies = \[[^\]]*'book-playback-timeline'[^\]]*\]/.test(sentenceQueueSource) && !/this\.dependencies = \[[^\]]*'sentence-queue'[^\]]*\]/.test(bookPlaybackTimelineSource) && /calculateAnimationTiming\(words = \[\]/.test(bookPlaybackTimelineSource)],
     ['3D display playback is owned by book playback timeline', /book-playback-timeline/.test(uiDisplayHandlerSource) && /playWebGLBookSentence/.test(uiDisplayHandlerSource) && /timeline\.playSentence\(sentence\)/.test(uiDisplayHandlerSource) && /if \(useWebGLBookReveal\) \{[\s\S]*await this\.playWebGLBookSentence\(sentence\);[\s\S]*this\.markBlockRendered\(sentence\.blockId\);[\s\S]*return null;/.test(uiDisplayHandlerSource)],