Move page rasterization to an OffscreenCanvas worker

Page text drawing (the bulk of drawSpread cost: layout, fonts, fillText across ~25 lines
x 2 pages at 3072px) ran synchronously on the main thread during prepare/lookahead, tanking
FPS at load and at flips/word boundaries.

New public/js/book-texture-worker.js owns rasterization off-thread: it loads the EB Garamond
faces via FontFace, draws base + title + lines + page number into an OffscreenCanvas, and
returns a full-page ImageBitmap plus a background-only base ImageBitmap (for the reveal mask)
per side. The main thread blits those onto the existing page canvases with one drawImage, so
the texture/reveal/scene pipeline downstream is unchanged. The worker also owns image loading
(fetch + createImageBitmap) and a DOM-free inline-tag parser (no document in a worker); the
renderer marshals the DOM-sourced title data in.

drawSpread is now async and serialized through a promise chain so the shared render state
(currentSpread, revealPublishBlockIds, spread override, reveal base) stays consistent across
the worker round trip even with concurrent lookahead prepares; the reveal context is passed
per draw rather than left on the instance. prepareRevealBlock / prepareContinuationRevealPlan /
preloadAdditionalRevealSpreads and their timeline callers await accordingly. The old
main-thread drawing methods are deleted (single implementation now lives in the worker).

Verified live: pages render correctly via the worker (text + drop caps crisp), worker fonts
load (probe returns fonts-ready + drawn), idle ~66fps, playback median ~60fps. Remaining
non-rasterization main-thread costs (procedural texture generation in the loader; pagination
text layout; per-frame reflection/shadow on content change) are separate follow-ups. Suite 166.

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

scripts/check-webgl-book-lab.js

@@ -37,6 +37,8 @@ const bookPlaybackTimelinePath = path.join(__dirname, '..', 'public', 'js', 'boo
 const bookPlaybackTimelineSource = fs.readFileSync(bookPlaybackTimelinePath, 'utf8');
 const ttsFactoryPath = path.join(__dirname, '..', 'public', 'js', 'tts-factory-module.js');
 const ttsFactorySource = fs.readFileSync(ttsFactoryPath, 'utf8');
+const textureWorkerPath = path.join(__dirname, '..', 'public', 'js', 'book-texture-worker.js');
+const textureWorkerSource = fs.readFileSync(textureWorkerPath, 'utf8');
 
 function dependencyList(source, moduleId) {
     const classStart = source.indexOf(`super('${moduleId}'`);
@@ -160,7 +162,8 @@ const checks = [
     ['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)],
-    ['texture renderer paints inline bold and italic styles', /getInlineStyleState/.test(textureRendererSource) && /updateInlineStyleState/.test(textureRendererSource) && /getCanvasFont/.test(textureRendererSource) && /segment\?\.style/.test(textureRendererSource)],
+    ['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)],
     ['webgl lab can preload page textures without swapping visible page material through texture store', /preparePageTexture\(side = 'left'/.test(webglPageCacheSource) && /takePreparedPageTexture\(side = 'left'/.test(webglPageCacheSource) && /renderer\.initTexture\(texture\)/.test(webglPageCacheSource) && /takePreparedPageTexture/.test(source) && !/const preparedPageTextures/.test(source)],
     ['webgl page text textures avoid mipmap generation', /function configurePageCanvasTexture/.test(source) && /texture\.minFilter = THREE\.LinearFilter/.test(source) && /texture\.generateMipmaps = false/.test(source)],
     ['webgl reveal shader masks against a base-page texture instead of flat color blocks', /bookRevealBaseMap/.test(source) && /bookRevealUseBaseMap/.test(source) && /revealBaseColor/.test(source) && /baseCanvas/.test(textureRendererSource)],
@@ -181,12 +184,12 @@ const checks = [
     ['pagination normalizes every spread to explicit left and right page records', /normalizePagesForSpreads/.test(bookPaginationSource) && /const lastSpreadRightIndex/.test(bookPaginationSource) && /this\.createBlankPage\(index/.test(bookPaginationSource) && /normalizedPages\.forEach/.test(bookPaginationSource)],
     ['texture renderer adopts initial pagination spread so title page is painted after loader order', /this\.currentSpread = this\.pagination\?\.getCurrentSpread\?\.\(\) \|\| \{ index: 0/.test(textureRendererSource) && /this\.drawSpread\(this\.currentSpread\);/.test(textureRendererSource)],
     ['pagination publishes page metadata and advances near the end of physical flips', /pageMeta/.test(bookPaginationSource) && /webgl-book:page-flip-near-end/.test(bookPaginationSource) && /this\.setCurrentSpread\(this\.currentSpreadIndex \+ direction\)/.test(bookPaginationSource)],
-    ['texture renderer draws title page and page numbers from versioned page metadata', /drawTitlePage/.test(textureRendererSource) && /game_title/.test(textureRendererSource) && /drawPageNumber/.test(textureRendererSource) && /pageMeta: this\.buildPublishPageMeta\(sidesToPublish\)/.test(textureRendererSource)],
-    ['texture renderer uses plural page margin metrics for page numbers', /this\.metrics\.margins\.bottom/.test(textureRendererSource) && !/this\.metrics\.margin\.bottom/.test(textureRendererSource)],
+    ['texture worker draws title page and page numbers; renderer marshals title data and versioned page metadata', /drawTitlePage/.test(textureWorkerSource) && /drawPageNumber/.test(textureWorkerSource) && /game_title/.test(textureRendererSource) && /buildTitleData/.test(textureRendererSource) && /pageMeta: this\.buildPublishPageMeta\(sidesToPublish\)/.test(textureRendererSource)],
+    ['texture worker uses plural page margin metrics for page numbers', /metrics\.margins\.bottom/.test(textureWorkerSource) && !/metrics\.margin\.bottom/.test(textureWorkerSource)],
     ['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) && /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 = await 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)],
@@ -226,7 +229,7 @@ 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 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)],
+    ['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)],
     ['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)],