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>

docs/webgl-3d-ui-spec.md

@@ -52,40 +52,90 @@ This section records the current state after the procedural book integration wor
 - The custom book material shader includes a small local indirect/bounce-light term for book surfaces. This is not emissive; it is multiplied by material albedo so paper, leather, and head/end-band colors remain distinct while shadowed side faces stay readable.
 - Temporary screenshots and generated debug images are not product assets unless explicitly promoted.
 
-## Current Page-Flow Architecture
+## Page-Flow Architecture
 
-The 3D book pipeline is module-owned. No page content should be generated by ad hoc scene code.
+The 3D book is the primary display. A secondary 2D DOM view may be driven from the same content,
+but it is never the source of truth and must not own playback decisions. No page content is
+generated by ad hoc scene code.
 
-- `ui-display-handler` owns the live 3D prepare -> optional page flip -> activate sequence for story text.
-- `sentence-queue` may prepare future page presentations during lookahead using the same pagination and texture renderer modules.
-- `book-pagination` owns page/spread construction, page metadata, widows/orphans/hyphenation decisions, image placement decisions, and explicit blank/title/body page records.
-- `book-texture-renderer` owns drawing final page canvases, reveal-region coordinates, reveal timing metadata, and publishing explicit `webgl-book:page-texture-records`.
-- `webgl-page-cache` owns persistent page canvases, memory canvases, prepared reveal plans, prepared GPU textures, resident VRAM textures, blank page texture, and visible texture bindings.
-- `webgl-book-lab` owns the Three.js scene, materials, geometry, pointer projection, page flip meshes, and consuming page texture records. It must not become a second page cache.
+### Single ownership
 
-Problem states must be surfaced instead of hidden:
+There is exactly one playback owner: `book-playback-timeline`. It owns the complete content
+lifecycle for story text and is the only module allowed to sequence it:
+
+```
+prepare (pagination + textures + prewarm)
+  -> activate (make the target spread the visible spread)
+  -> reveal   (animate the new block's text in)
+  -> flip     (turn the page when a spread boundary is crossed)
+```
+
+- `ui-display-handler` and `sentence-queue` are clients of the owner. They call
+  `book-playback-timeline.playSentence` (live) and `prepareSentence` (lookahead). They contain no
+  flip, reveal, or spread-transition logic of their own.
+- `book-pagination` owns page/spread construction, page metadata, widows/orphans/hyphenation,
+  image placement, and explicit blank/title/body page records. It does not decide playback timing.
+- `book-texture-renderer` owns drawing final page canvases and computing reveal-region coordinates
+  and per-region timing metadata. It is a pure renderer: it does **not** run a playback clock, does
+  **not** decide when reveals start/finish, and does **not** trigger flips. Reactive redraws keyed
+  off pagination events are forbidden; the owner asks it to draw.
+- `webgl-page-cache` is the single texture/canvas cache: persistent canvases, memory canvases,
+  prepared reveal plans, prepared GPU textures, resident VRAM textures, the blank texture, and
+  visible texture bindings. No other module may keep a parallel cache.
+- `webgl-book-scene` (implemented in `webgl-book-lab.js`) owns the Three.js scene, materials,
+  geometry, pointer projection, page-flip meshes, the single reveal clock, and consuming page
+  texture records. It must not become a second page cache and must not decide playback order.
+
+### Single reveal clock
+
+Reveal timing has exactly one authority: the scene render loop. It advances reveal progress,
+freezes it during a physical flip, and emits `webgl-book:reveal-committed` when a side's reveal
+finishes. No module runs a second `setTimeout`/`requestAnimationFrame` reveal clock in parallel.
+The texture renderer supplies region timings; it does not measure their elapsed time.
+
+### Owner-to-scene command channel
+
+The owner drives the scene through the registered scene command interface obtained from the module
+registry (`webgl-book-scene`), or through the formal `webgl-book:*` events listed below. The owner
+must never reach into `window.BookLabDebug` — that object is a debug/inspection surface only and is
+not part of any production control path. Production code must not throw because a debug global is
+missing.
+
+### Problem states are surfaced, never hidden
 
 - A missing persistent page canvas during prewarm is a `db-cache-miss`.
-- A missing source or required back texture before a page flip is a `flip-source-texture-missing` or `flip-back-texture-missing`.
-- These problem states must appear in `webglPageCacheProblems` and must not be silently fixed by borrowing unrelated visible stack textures.
+- A missing source or required back texture before a page flip is a `flip-source-texture-missing`
+  or `flip-back-texture-missing`.
+- These appear in `webglPageCacheProblems` and must not be silently fixed by borrowing unrelated
+  visible stack textures. Surfacing a problem must not abort live reading: a transient miss degrades
+  (blank/last-good texture) and is logged, rather than throwing out of the playback path.
 
 ## Event Surface
 
-The preferred 3D book content path is direct module calls through `ui-display-handler` for live playback and `sentence-queue` for lookahead preparation.
+The owner controls the scene through the scene command interface (preferred for direct,
+ordered calls) and through these formal events. Each event has one producer and stable meaning:
 
-The following events remain formal integration events and must keep their meaning stable while they exist:
+- `webgl-book:page-texture-records` (owner/renderer -> scene): publishes explicit page texture
+  records for a spread, carrying `phase` (`prepare`|`activate`) and per-side `visibility`.
+- `webgl-book:page-reveal-start` (owner -> scene): starts the scene reveal clock for a block.
+- `webgl-book:page-reveal-fast-forward` (owner -> scene): accelerates reveal timing without
+  replacing the page pipeline.
+- `webgl-book:reveal-committed` (scene -> owner): a page-side reveal completed. The owner — not the
+  scene — decides whether a flip follows.
+- `webgl-book:request-page-flip` (owner -> scene): requests a physical page flip.
+- `webgl-book:page-flip-started`, `webgl-book:page-flip-near-end`, `webgl-book:page-flip-finished`
+  (scene -> owner): the physical flip lifecycle, each carrying the resolved `targetSpread`.
 
-- `webgl-book:page-texture-records` publishes explicit page texture records from `book-texture-renderer` to the WebGL scene.
-- `webgl-book:page-reveal-start` starts shader reveal timing for the prepared block.
-- `webgl-book:page-reveal-fast-forward` accelerates reveal timing without replacing the page pipeline.
-- `webgl-book:reveal-committed` reports that a page-side reveal completed; if `pageFlipAfterReveal` is true, the WebGL scene may arm a page flip.
-- `webgl-book:request-page-flip` requests a physical page flip through the WebGL scene.
-- `webgl-book:page-flip-started`, `webgl-book:page-flip-near-end`, and `webgl-book:page-flip-finished` describe the physical flip lifecycle.
+The scene reacts to these events; it does not originate flip decisions from `reveal-committed`.
 
-Deprecated or forbidden event contracts:
+Deprecated or forbidden contracts:
 
-- `webgl-book:page-canvases` is obsolete. New code must use `webgl-book:page-texture-records`.
-- `preloadOnly` and `allowFutureUnrendered` are obsolete boolean flags. New code must use explicit `phase` and `visibility` values.
+- `webgl-book:page-canvases` is obsolete; use `webgl-book:page-texture-records`.
+- `preloadOnly` and `allowFutureUnrendered` boolean flags are obsolete; use explicit `phase` and
+  `visibility` values.
+- The legacy `ownsPageFlipCommit` toggle and the `book-pagination:spread-updated`-driven reveal/flip
+  path in `book-texture-renderer` and the scene are removed. There is no second playback path to
+  gate, so no gating flag exists.
 
 ## Non-Negotiable Workflow Rules