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-worker.js

@@ -9,6 +9,10 @@
 let fontsReady = null;
 const imageCache = new Map(); // src -> ImageBitmap | null
 const surfaces = {};          // side -> { canvas, ctx }
+// The reveal "base" layer is the plain paper background (drawPageBase) — identical for every
+// page of a side at a given size. Send its bitmap only once per side+size; the main thread
+// caches and reuses it, avoiding a large per-block ImageBitmap allocation (GC churn).
+const sentBaseKeys = new Set();
 
 function resolveImageSource(metadata = {}) {
     const explicit = String(metadata.url || metadata.src || '').trim();
@@ -319,7 +323,11 @@ async function renderSide(job, side) {
 
     drawPageBase(ctx, side, width, height);
     let baseBitmap = null;
-    if (job.hasReveal) baseBitmap = await createImageBitmap(surface.canvas);
+    const baseKey = `${side}:${width}x${height}`;
+    if (job.hasReveal && !sentBaseKeys.has(baseKey)) {
+        baseBitmap = await createImageBitmap(surface.canvas);
+        sentBaseKeys.add(baseKey);
+    }
     if (meta?.kind === 'title') drawTitlePage(ctx, metrics, side, job.titleData);
     drawPageLines(ctx, metrics, side, job.spreads?.[side] || []);
     drawPageNumber(ctx, metrics, side, meta);