Fix page label and restore nav code after bad clone-removal revert

The previous commit's clone removal corrupted the flip (shared live canvas overwritten mid-
animation -> flicker); reverting it fixed the flip but also discarded code that had ridden into
that commit and broke the page label. Recovering:

- Page label: the right-page pageNumber lookup returned null (meta not populated for the queried
  index) so every spread read 0. Now derive the printed number from the index (frontmatter
  pages 0-2 are unnumbered, so right-page index N prints as N-2), preferring the paginated
  pageNumber when present. Title still reads 0.
- Restored the manual-navigation-busy guard and the written-content navigation cap (no flipping
  forward into blank leaves before content exists; title stays on its own spread).

The flip flicker fix is the clone restoration in the prior revert; this restores the label and
navigation behavior on top of it. Suite 182. Flip-flicker and per-paragraph stutter still need
verification on a real (non-throttled) foreground tab.

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

scripts/check-webgl-book-lab.js

@@ -258,7 +258,8 @@ const checks = [
     ['book playback timeline flips at planned right-page fragment time without a stray commit timeout', /waitForPlannedRightReveal/.test(bookPlaybackTimelineSource) && /getRightRevealDurationMs/.test(bookPlaybackTimelineSource) && /segment\.revealStartedPromise/.test(bookPlaybackTimelineSource) && /const timer = setTimeout\(\(\) => finish\(true\), remaining\)/.test(bookPlaybackTimelineSource) && !/waitForRevealCommit/.test(bookPlaybackTimelineSource)],
     ['book playback timeline exposes reveal lifecycle benchmark entries', /benchmarkEntries/.test(bookPlaybackTimelineSource) && /markBenchmark\('reveal-start'/.test(bookPlaybackTimelineSource) && /markBenchmark\('reveal-committed'/.test(bookPlaybackTimelineSource) && /webglBookBenchmark/.test(bookPlaybackTimelineSource)],
     ['webgl scene records reveal start and slow-frame benchmark diagnostics', /revealState:created/.test(source) && /revealStart:applied/.test(source) && /slowFrameLog/.test(source) && /getBenchmarkState/.test(source) && /webglSlowFrames/.test(source)],
-    ['webgl navigation is spread-based and caps at visited/written spread', /function navigateToSpread\(/.test(source) && /function navigateBySpreadDelta\(/.test(source) && /function getMaxNavigableSpread\(\)/.test(source) && /Math\.min\(visitedSpread, spreadCount - 1\)/.test(source) && /navigateBySpreadDelta\(1\)/.test(source) && /spread <= 0 \? '0' : String\(spread \* 2 \+ 1\)/.test(source) && /currentSpread < getMaxNavigableSpread\(\)/.test(source)],
+    ['webgl navigation is spread-based and caps at the written-content spread (title-only before content)', /function navigateToSpread\(/.test(source) && /function navigateBySpreadDelta\(/.test(source) && /function getMaxNavigableSpread\(\)/.test(source) && /Math\.min\(visitedSpread, contentSpread, spreadCount - 1\)/.test(source) && /writtenPageLimit >= 3 \? pageToSpreadIndex\(writtenPageLimit\) : 0/.test(source) && /navigateBySpreadDelta\(1\)/.test(source) && /currentSpread < getMaxNavigableSpread\(\)/.test(source)],
+    ['webgl spread label reads 0 at the title and the right page number elsewhere', /function spreadPageLabel\(spreadIndex\)/.test(source) && /if \(spread <= 0\) return '0'/.test(source) && /spreadPageIndices\(spread\)\.right/.test(source) && /rightPageIndex - 2/.test(source)],
     ['webgl manual page navigation is blocked while reveal playback or flips are active', /function isManualBookNavigationBusy\(\) \{[\s\S]*activeFlips\.length > 0[\s\S]*hasActivePageReveal\(\)[\s\S]*webglBookPlaybackActive/.test(source) && /function navigateToSpread\(targetSpread\) \{[\s\S]*if \(isManualBookNavigationBusy\(\)\) \{[\s\S]*navigation:blocked-busy/.test(source) && /bottomNavigation\.slider\.disabled = busy/.test(source)],
     ['webgl fast-forward always reaches scene reveal state even without renderer-side active animations', /fastForwardAnimations\(\) \{[\s\S]*webgl-book:page-reveal-fast-forward[\s\S]*broad: !changed/.test(textureRendererSource) && /function fastForwardPageReveals\(blockIds = \[\]\) \{[\s\S]*const matches = ids\.size === 0/.test(source)],
     ['webgl save restore carries visited page limit for navigation', /maxVisitedPagePosition/.test(source) && /setMaxVisitedPagePosition/.test(source) && /state\.maxVisitedPagePosition \?\? state\.pagePosition/.test(webglSceneSource)],