Harden worker page rendering: error/timeout recovery and awaited flip prewarm

Two robustness gaps from the worker migration, both raised in review:

- The raster worker had no failure recovery: a thrown createImageBitmap/font error or a
  dropped message would leave the draw promise pending forever, stalling the serialized draw
  chain and hanging prepare/playback. Added worker.onerror and a per-job timeout; both settle
  the in-flight draw to a logged miss (texture-worker-error / -timeout) so the pipeline
  degrades to last-good per the spec instead of hanging. A single settleRasterization path
  clears the timer and resolves.
- prepareSpreadTextureRecordsForFlip() called drawSpread() without awaiting it. That was safe
  when drawSpread was synchronous, but now that it is async (worker) the flip could race ahead
  of the worker draw and miss the resident texture. prewarmFlipTextures now awaits both spread
  draws before the resident-texture lookup.

Suite 168 (added assertions for worker error/timeout recovery and the awaited prewarm).
Normal draw path is behaviorally unchanged from the verified worker commit.

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

scripts/check-webgl-book-lab.js

@@ -164,6 +164,8 @@ const checks = [
     ['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 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)],
+    ['flip prewarm awaits the async worker draw before the resident-texture lookup', /await prepareSpreadTextureRecordsForFlip\(currentSpread\)/.test(source) && /await window\.BookTextureRenderer\.drawSpread\(spread, \['left', 'right'\]/.test(source)],
     ['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)],