Spanning playback: one background-prepared path, no fallbacks

For a block that overflows onto the next spread, the plan is now prepared spanning-aware
during the background lookahead — the start spread's reveal timing is derived across both
preview spreads, and the continuation spread's plan is prepared and cached at the same time.

playback then follows a single path:
- activate reuses the prepared start plan (removed the synchronous forceRebuild rebuild).
- revealContinuationSpread reuses the prepared continuation plan (removed the redraw
  fallback); a missing plan is surfaced as a problem, not silently redrawn.

This removes the parallel/immediate prepare distinction and the two fallbacks, leaving one
intended path, and moves the spanning draw work off the critical path.

Verified live on a real spanning block: right line reveals at its area share (~3.3s), the
flip fires, and the continuation appears ~0.3s after the flip (was ~2.7s) and animates
progressively across the next spread over the full TTS — no pop-in, no fast-forward, no
timeline-reveal-continuation-missing. Static suite passes (165).

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

scripts/check-webgl-book-lab.js

@@ -226,7 +226,7 @@ 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)],
+    ['book playback timeline has one prepare path: spanning-aware plan reused at activate and continuation, no fallback', /this\.revealSpreadSourceOverride = detail\.previewSpreads/.test(textureRendererSource) && !/forceRebuild/.test(textureRendererSource) && !/forceRebuild/.test(bookPlaybackTimelineSource) && !/spanningPlanPrepared/.test(bookPlaybackTimelineSource) && /const texturePlan = this\.textureRenderer\.takeContinuationRevealPlan\(segment\.blockId, spread\.index\)/.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)],