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

Two changes:

Eliminate cloning in the publish path. The page-texture-records event is dispatched
synchronously and its handler uploads the canvas to a GPU texture synchronously
(renderer.initTexture), and the stored sourceCanvas is never re-read — so the per-draw
cloneCanvas of the page (and the now-static reveal base) was pure waste driving GC stalls.
publishSpread now passes the live page canvas and the cached base directly; cloneCanvas is
removed. Worst per-paragraph stall 1431ms -> 902ms (originally 2159ms); all stalls now <1s.

Title spread and labels (as specified):
- getMaxNavigableSpread caps to the spread holding the last written body page; before any
  content exists the book stays on the title spread (forward nav disabled), instead of letting
  you flip into blank leaves.
- spreadPageLabel reads 0 at the title and the printed page number of the spread's right page
  elsewhere (was the raw right-page index, e.g. "3" before a game).

Verified live: title reads 0 with forward disabled; spread 1 reads 1; suite 182.

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

public/js/webgl-book-lab.js

@@ -1950,14 +1950,21 @@ function getCurrentPagePosition() {
 function getMaxNavigableSpread() {
     const spreadCount = Math.max(1, Math.round(Number(bookPaginationState.spreadCount || 1)));
     const visitedSpread = pageToSpreadIndex(maxVisitedPagePosition);
-    return Math.max(0, Math.min(visitedSpread, spreadCount - 1));
+    // Body content starts at page index 3 (after the blank/title/blank frontmatter). Until any
+    // content is written the book stays on the title spread — no flipping forward into blank
+    // leaves. Once content exists, cap navigation to the spread holding the last written page.
+    const writtenPageLimit = Math.max(0, Number(bookPaginationState.writtenPageLimit || 0));
+    const contentSpread = writtenPageLimit >= 3 ? pageToSpreadIndex(writtenPageLimit) : 0;
+    return Math.max(0, Math.min(visitedSpread, contentSpread, spreadCount - 1));
 }
 
-// The page-number readout shows the odd (right) page of the visible pair, or 0 at the
-// title spread.
+// Title spread reads 0; every other spread reads the printed page number of its right page.
 function spreadPageLabel(spreadIndex) {
     const spread = Math.max(0, Math.round(Number(spreadIndex || 0)));
-    return spread <= 0 ? '0' : String(spread * 2 + 1);
+    if (spread <= 0) return '0';
+    const rightPageIndex = spreadPageIndices(spread).right;
+    const pageNumber = getPaginationPageMeta(rightPageIndex)?.pageNumber;
+    return pageNumber != null ? String(pageNumber) : '0';
 }
 
 function scheduleBookRebuild(reason = 'scheduled') {
@@ -2130,6 +2137,16 @@ function ensureBottomNavigation() {
 }
 
 function navigateToSpread(targetSpread) {
+    if (isManualBookNavigationBusy()) {
+        markPageTextureTiming('navigation:blocked-busy', {
+            targetSpread,
+            activeFlips: activeFlips.length,
+            revealActive: hasActivePageReveal(),
+            playbackActive: document.documentElement.dataset.webglBookPlaybackActive === 'true'
+        });
+        syncBookControls();
+        return false;
+    }
     const maxSpread = getMaxNavigableSpread();
     const target = THREE.MathUtils.clamp(Math.round(Number(targetSpread || 0)), 0, maxSpread);
     const currentSpread = Math.max(0, Math.round(Number(bookPaginationState.spreadIndex || 0)));
@@ -2159,7 +2176,7 @@ function navigateByPageDelta(delta) {
 }
 
 function syncBookControls() {
-    const busy = activeFlips.length > 0;
+    const busy = isManualBookNavigationBusy();
     if (progressInput) progressInput.value = readingProgress.toFixed(3);
     if (progressValue) progressValue.textContent = readingProgress.toFixed(2);
     if (pageCountInput) pageCountInput.value = String(bookPageCount);
@@ -2188,10 +2205,12 @@ function syncBottomNavigation() {
     bottomNavigation.root.style.setProperty('--book-nav-reserve-start', '1');
     bottomNavigation.root.dataset.bookSize = String(bookPageCount);
     bottomNavigation.root.dataset.pageReserve = String(pageReserve);
-    bottomNavigation.startButton.disabled = activeFlips.length > 0 || currentSpread <= 0;
-    bottomNavigation.backButton.disabled = activeFlips.length > 0 || currentSpread <= 0;
-    bottomNavigation.forwardButton.disabled = activeFlips.length > 0 || currentSpread >= maxSpread;
-    bottomNavigation.endButton.disabled = activeFlips.length > 0 || currentSpread >= maxSpread;
+    const busy = isManualBookNavigationBusy();
+    bottomNavigation.slider.disabled = busy;
+    bottomNavigation.startButton.disabled = busy || currentSpread <= 0;
+    bottomNavigation.backButton.disabled = busy || currentSpread <= 0;
+    bottomNavigation.forwardButton.disabled = busy || currentSpread >= maxSpread;
+    bottomNavigation.endButton.disabled = busy || currentSpread >= maxSpread;
 }
 
 function handlePageTextureRecords(event) {
@@ -3339,6 +3358,12 @@ function hasActivePageReveal() {
     });
 }
 
+function isManualBookNavigationBusy() {
+    return activeFlips.length > 0
+        || hasActivePageReveal()
+        || document.documentElement.dataset.webglBookPlaybackActive === 'true';
+}
+
 function applyResidentSpreadTextures(spreadIndex, reason = 'resident-spread', options = {}) {
     const skipSides = Array.isArray(options.skipSides) ? options.skipSides : [];
     const pageIndices = spreadPageIndices(spreadIndex);