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>

public/js/book-texture-renderer-module.js

@@ -336,10 +336,17 @@ class BookTextureRendererModule extends BaseModule {
             ctx.clearRect(0, 0, this.canvases[side].width, this.canvases[side].height);
             ctx.drawImage(result.pageBitmap, 0, 0);
             result.pageBitmap.close?.();
-            if (hasReveal && result.baseBitmap) {
-                this.revealBaseCanvases[side] = this.canvasFromBitmap(result.baseBitmap);
+            // The paper base is identical for every page of a side; the worker sends its bitmap
+            // only once, and we cache the canvas and reuse it for all reveals. This removes a
+            // large per-block canvas/bitmap allocation that was driving GC stalls.
+            if (result.baseBitmap) {
+                if (!this.cachedBaseCanvas) this.cachedBaseCanvas = {};
+                this.cachedBaseCanvas[side] = this.canvasFromBitmap(result.baseBitmap);
+                result.baseBitmap.close?.();
+            }
+            if (hasReveal) {
+                this.revealBaseCanvases[side] = this.cachedBaseCanvas?.[side] || null;
             }
-            result.baseBitmap?.close?.();
         });
         const published = this.publishSpread(sidesToDraw, options);
         this.markPipelineTiming('drawSpread:end', { sides: sidesToDraw, phase });