Make WebGL book navigation spread-based and clear stale flip reveal mask

- Navigation now operates on spread indices, not the non-contiguous page-position
  scheme that mapped a forward step onto the same spread (so forward stalled and
  triggered a no-op multi-flip). Forward/back move one spread; start/end and the
  slider use spread indices. The page readout shows the odd page of the visible
  pair (2*spread+1) or 0 at the title spread.
- Flipping forward could show the source page with its last word still masked: a
  stale reveal mask left on the flip surface by a previous playback flip was not
  cleared when the (finished) source page had no active reveal. Reset the flip
  surface reveal shader in that case so the full page shows during the turn.

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

scripts/check-webgl-book-lab.js

@@ -240,7 +240,7 @@ const checks = [
     ['book playback timeline flips at planned right-page fragment time instead of full TTS completion', /waitForPlannedRightReveal/.test(bookPlaybackTimelineSource) && /getRightRevealDurationMs/.test(bookPlaybackTimelineSource) && /segment\.revealStartedPromise/.test(bookPlaybackTimelineSource) && /Promise\.race\(\[[\s\S]*this\.waitForRevealCommit\(segment\)/.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 buttons cap at visited page and written content limit', /maxVisitedPagePosition/.test(source) && /navigateToPagePosition\(maxVisitedPagePosition\)/.test(source) && /function getNavigablePageLimit\(\)/.test(source) && /const navigableLimit = getNavigablePageLimit\(\)/.test(source) && /Math\.min\(maxVisitedPagePosition, getWritablePageLimit\(\), contentNavigable\)/.test(source) && !/navigateToPagePosition\(bookPaginationState\.writtenPageLimit\)/.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 save restore carries visited page limit for navigation', /maxVisitedPagePosition/.test(source) && /setMaxVisitedPagePosition/.test(source) && /state\.maxVisitedPagePosition \?\? state\.pagePosition/.test(webglSceneSource)],
     ['webgl page flips require resident nonblank back textures before animation starts', /prepareStaticPageForFlip\(flip, prewarm = null\)/.test(source) && /flip-back-texture-missing/.test(source) && /targetBackPageMeta\.kind !== 'blank'/.test(source) && /return false;/.test(methodBody(source, 'prepareStaticPageForFlip')) && /flipTexturePreflight:ready/.test(source) && /if \(!prepareStaticPageForFlip\(flip, options\.prewarm \|\| null\)\) \{[\s\S]*return false;[\s\S]*\}/.test(source)],
     ['webgl fast page flips preflight the actual target spread', /firstFlip\.targetSpread = Number\.isFinite\(Number\(options\.targetSpread\)\)/.test(source) && /if \(!prepareStaticPageForFlip\(firstFlip, options\.prewarm \|\| null\)\) return false/.test(source)],