Fix new-game title flip + cap lookahead prepare burst

Builds on the worker migration with prepare-burst pacing and a title-flip fix:

- New game from mid-game left the book on the previous game's spread, so the first block's
  source and target spread matched and the title->content page turn was skipped. story:client-reset
  now returns the book to the title spread (spread 0) so the first block flips 0->1 and animates.
  Verified: requiresSpreadTransition src=0 tgt=1, page-flip-started/near-end fire.

- The lookahead burst-prepared many blocks at once, spiking allocation/GC into multi-second
  main-thread stalls. WebGL book prepares are now serialized through a chain and capped to a
  small lookahead window (TTS audio prefetch still spans the full window); future lookahead is
  also deferred until the current sentence has entered the display pipeline, keeping it off the
  first flip/reveal critical path. Worst game-start stall ~6s -> ~3.4s.

- Page flips now drive the scene through the sceneControl prewarm/startPreparedPageFlip API
  (awaited) instead of an event, and the scene awaits the async initial spread draw.

Suite 177. Remaining: a per-block prepare stall (~1.6-3.4s for large blocks at game start)
that profiling has not yet attributed to a single function (likely GC from prepare-path
allocation) — needs a DevTools performance capture for exact attribution.

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

public/js/webgl-book-scene-module.js

@@ -365,14 +365,14 @@ class WebGLBookSceneModule extends BaseModule {
 
     async initializeScene() {
         if (this.labImportPromise) return this.labImportPromise;
-        const cacheBuster = window.MODULE_CACHE_BUSTER || Date.now();
-        this.labImportPromise = import(`/js/webgl-book-lab.js?v=${encodeURIComponent(cacheBuster)}`);
+        const moduleVersion = window.MODULE_CACHE_BUSTER || 'dev';
+        this.labImportPromise = import(`/js/webgl-book-lab.js?v=${encodeURIComponent(moduleVersion)}`);
         await this.labImportPromise;
         this.reportProgress(94, 'Uploading initial book page textures');
         const pagination = this.getModule('book-pagination');
         const initialSpread = pagination?.getCurrentSpread?.();
         if (initialSpread && typeof window.BookTextureRenderer?.drawSpread === 'function') {
-            window.BookTextureRenderer.drawSpread(initialSpread, ['left', 'right'], { force: true });
+            await window.BookTextureRenderer.drawSpread(initialSpread, ['left', 'right'], { force: true });
         } else {
             window.BookTextureRenderer?.publishSpread?.();
         }