Cut per-paragraph GC stalls: reuse static paper base, cap lookahead to 1

Profiling the per-paragraph playback stutter showed the JS heap sawtoothing (37<->71MB) with
0.4-2.2s long tasks once per block — GC pauses from large (24-48MB) per-block canvas/ImageBitmap
allocations, not pagination (buildPages was ~29ms). These pauses freeze the flip/reveal
animation, which is also why the title flip looked un-animated.

- The reveal "base" layer is the plain paper background, identical for every page of a side.
  The worker now sends its bitmap once per side+size; the renderer caches the canvas and reuses
  it for all reveals, removing a large per-block bitmap+canvas allocation.
- WEBGL_BOOK_PREFETCH_LOOKAHEAD 2 -> 1 so only the next block's page render is prepared, instead
  of letting multiple large rasterizations overlap.

Verified live: per-paragraph long tasks roughly halved (10 -> 5 over the same window) and worst
case 2159ms -> 1431ms. Residual ~1.4s stall remains from the per-block page bitmap + prepared-
page snapshot clone + texture upload; further reduction needs reworking those to reuse buffers.
Suite 181.

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

scripts/check-webgl-book-lab.js

@@ -145,7 +145,8 @@ const checks = [
     ['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 caps WebGL book lookahead without capping TTS lookahead window', /const WEBGL_BOOK_PREFETCH_LOOKAHEAD = 1/.test(sentenceQueueSource) && /webglBookLookahead < WEBGL_BOOK_PREFETCH_LOOKAHEAD/.test(sentenceQueueSource) && !/spokenPrepared >= 1 && started >= 2/.test(sentenceQueueSource)],
+    ['texture worker sends the static paper base bitmap once per side and the renderer reuses it', /sentBaseKeys/.test(textureWorkerSource) && /const baseKey = `\$\{side\}:\$\{width\}x\$\{height\}`/.test(textureWorkerSource) && /this\.cachedBaseCanvas\[side\] = this\.canvasFromBitmap/.test(textureRendererSource) && /this\.revealBaseCanvases\[side\] = this\.cachedBaseCanvas\?\.\[side\]/.test(textureRendererSource)],
     ['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)],