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>

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) && /getPaginationPageMeta\(rightPageIndex\)\?\.pageNumber/.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)],