Revert "Remove per-draw canvas clones; title nav cap + right-page-number labels"

This reverts commit 0f66dae4eb.

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

@@ -53,6 +53,7 @@ class BookTextureRendererModule extends BaseModule {
             'drawSpreadSerial',
             'rasterizeSpread',
             'getDrawSignature',
+            'cloneCanvas',
             'buildRevealRegions',
             'shouldFlipAfterSideReveal',
             'collectRevealRegionCandidates',
@@ -372,6 +373,15 @@ class BookTextureRendererModule extends BaseModule {
         }).join('|');
     }
 
+    cloneCanvas(canvas) {
+        if (!canvas) return null;
+        const clone = document.createElement('canvas');
+        clone.width = canvas.width;
+        clone.height = canvas.height;
+        const context = clone.getContext('2d');
+        if (context) context.drawImage(canvas, 0, 0);
+        return clone;
+    }
 
     getPageContent(side = 'left') {
         return this.metrics?.contentBySide?.[side] || this.metrics?.content || {
@@ -943,17 +953,19 @@ class BookTextureRendererModule extends BaseModule {
             pageMeta: this.buildPublishPageMeta(sidesToPublish),
             phase
         };
-        // The page-texture-records event is dispatched synchronously below, and its handler
-        // uploads the canvas to a GPU texture synchronously (renderer.initTexture). So the live
-        // page canvas (and the static cached base) are consumed before the next draw can touch
-        // them — no per-draw clones needed (those large canvas copies drove the GC stalls).
-        if (sidesToPublish.includes('left')) detail.left = this.canvases.left;
-        if (sidesToPublish.includes('right')) detail.right = this.canvases.right;
+        if (sidesToPublish.includes('left')) {
+            detail.left = phase === 'prepare' ? this.cloneCanvas(this.canvases.left) : this.canvases.left;
+        }
+        if (sidesToPublish.includes('right')) {
+            detail.right = phase === 'prepare' ? this.cloneCanvas(this.canvases.right) : this.canvases.right;
+        }
         const reveal = {};
         sidesToPublish.forEach((side) => {
             const sideReveal = this.buildRevealRegions(side);
             if (!sideReveal) return;
-            sideReveal.baseCanvas = this.revealBaseCanvases?.[side] || null;
+            sideReveal.baseCanvas = phase === 'prepare'
+                ? this.cloneCanvas(this.revealBaseCanvases?.[side])
+                : this.revealBaseCanvases?.[side] || null;
             regionCounts[side] = sideReveal.lineRects.length;
             reveal[side] = sideReveal;
         });