Rework WebGL book playback to single ownership and fix flip/reveal pipeline

Establish book-playback-timeline as the sole playback owner driving the
scene through formal webgl-book:* events (not the BookLabDebug surface),
with a single reveal clock in the scene render loop and webgl-page-cache as
the only texture cache. Remove the legacy dual playback path and the
ownsPageFlipCommit gating.

Fixes:
- Flip page detached/folded at the spine: restore the raw page-cap line for
  flip geometry (matches the prototype/pre-regression), removing
  normalizeFlipLineToVisiblePage which moved the pivot off the spine arc.
- Flip textures: distance-based UVs (no horizontal compression),
  direction-aware face material (source on the camera-facing side), source
  meta derived from the visible spread (manual flips), prewarm shape fix.
- Reveal: flash removed on the static page and the flip back surface;
  spanning blocks rebuild the reveal plan at activate and continue the
  reveal on the next spread after the fill flip.
- Cache staleness is contentVersion-primary; nav clamps to spreadCount.

Docs updated to describe the intended single-owner architecture. Regression
checks updated to match.

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

public/js/webgl-book-scene-module.js

@@ -17,6 +17,10 @@ class WebGLBookSceneModule extends BaseModule {
         this.gameConfig = null;
         this.mode = '2d';
         this.is3dSupported = false;
+        // Production control surface + visible-spread accessor for the dynamically
+        // imported webgl-book-lab. Populated by the lab once the scene is built.
+        this.sceneControl = null;
+        this.getVisibleSpreadIndex = null;
         this.labImportPromise = null;
         this.textureRefreshTimer = null;
         this.textureRefreshAnimationId = null;
@@ -330,7 +334,7 @@ class WebGLBookSceneModule extends BaseModule {
                 this.persistenceManager?.updatePreference?.('webgl', 'pageReserve', value);
                 this.preferenceWriteGuard = false;
             },
-            getBookState: () => window.BookLabDebug?.getBookState?.() || {
+            getBookState: () => this.sceneControl?.getBookState?.() || {
                 pageCount: this.persistenceManager?.getPreference?.('webgl', 'bookPageCount', DEFAULT_BOOK_PAGE_COUNT) ?? DEFAULT_BOOK_PAGE_COUNT,
                 pageReserve: this.persistenceManager?.getPreference?.('webgl', 'pageReserve', DEFAULT_PAGE_RESERVE) ?? DEFAULT_PAGE_RESERVE,
                 progress: this.persistenceManager?.getPreference?.('webgl', 'bookProgress', DEFAULT_BOOK_PROGRESS) ?? DEFAULT_BOOK_PROGRESS
@@ -341,19 +345,19 @@ class WebGLBookSceneModule extends BaseModule {
                 const progress = Number(state.progress);
                 if (Number.isFinite(pageCount)) {
                     this.persistenceManager?.updatePreference?.('webgl', 'bookPageCount', pageCount);
-                    window.BookLabDebug?.setBookPageCount?.(pageCount);
+                    this.sceneControl?.setBookPageCount?.(pageCount);
                 }
                 if (Number.isFinite(pageReserve)) {
                     this.persistenceManager?.updatePreference?.('webgl', 'pageReserve', pageReserve);
-                    window.BookLabDebug?.setPageReserve?.(pageReserve);
+                    this.sceneControl?.setPageReserve?.(pageReserve);
                 }
                 if (Number.isFinite(progress)) {
                     this.persistenceManager?.updatePreference?.('webgl', 'bookProgress', progress);
-                    window.BookLabDebug?.setReadingProgress?.(progress);
+                    this.sceneControl?.setReadingProgress?.(progress);
                 }
                 const maxVisitedPagePosition = Number(state.maxVisitedPagePosition ?? state.pagePosition);
                 if (Number.isFinite(maxVisitedPagePosition)) {
-                    window.BookLabDebug?.setMaxVisitedPagePosition?.(maxVisitedPagePosition);
+                    this.sceneControl?.setMaxVisitedPagePosition?.(maxVisitedPagePosition);
                 }
             }
         };
@@ -456,7 +460,7 @@ class WebGLBookSceneModule extends BaseModule {
     }
 
     projectCanvasEventTarget(event) {
-        const projection = window.BookLabDebug?.projectPointerToPage?.(event.clientX, event.clientY);
+        const projection = this.sceneControl?.projectPointerToPage?.(event.clientX, event.clientY);
         if (!projection) {
             document.documentElement.dataset.webglLastProjection = JSON.stringify({
                 hit: false,
@@ -531,11 +535,11 @@ class WebGLBookSceneModule extends BaseModule {
                 this.initializeScene();
             }
         } else if (key === 'bookProgress' && !this.preferenceWriteGuard) {
-            window.BookLabDebug?.setReadingProgress?.(value);
+            this.sceneControl?.setReadingProgress?.(value);
         } else if (key === 'bookPageCount' && !this.preferenceWriteGuard) {
-            window.BookLabDebug?.setBookPageCount?.(value);
+            this.sceneControl?.setBookPageCount?.(value);
         } else if (key === 'pageReserve' && !this.preferenceWriteGuard) {
-            window.BookLabDebug?.setPageReserve?.(value);
+            this.sceneControl?.setPageReserve?.(value);
         }
     }
 
@@ -556,7 +560,7 @@ class WebGLBookSceneModule extends BaseModule {
     triggerTextureRefresh() {
         clearTimeout(this.textureRefreshTimer);
         this.textureRefreshTimer = setTimeout(() => {
-            window.BookLabDebug?.redrawPageTextures?.();
+            this.sceneControl?.redrawPageTextures?.();
         }, 60);
     }