Reset per-game reveal state on new game so reveal animates over cached content

Starting a new game reuses block ids (1,2,3...). The reveal clock's per-block
start times (activeRevealBlockStarts in the lab) and the renderer's animation/
revealed sets are keyed by block id and were never cleared on a client reset, so
a new game over already-cached content inherited the previous run's start times.
beginPageReveal then computed a huge elapsed and the shader treated the reveal as
already complete — showing everything at once instead of animating.

resetClientPlaybackAndDisplay (run on new game and restore) now emits
story:client-reset; the lab clears activeRevealBlockStarts/pending reveal state,
the texture renderer clears active animations and revealed-block ids, and the
timeline invalidates prepared segments. So each game starts with a clean reveal
clock.

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

public/js/webgl-book-lab.js

@@ -695,6 +695,15 @@ document.addEventListener('webgl-book:request-page-flip', (event) => {
 document.addEventListener('webgl-book:page-cache-problem', (event) => {
     pageTextureStore?.recordProblem?.(event.detail || {});
 });
+// New game / history restore: drop block-id-keyed reveal state so a reused block id does
+// not inherit the previous run's reveal start time (which would skip the animation).
+document.addEventListener('story:client-reset', () => {
+    activeRevealBlockStarts.clear();
+    pendingRevealStartBlockIds.clear();
+    pageRevealFreezeAt = null;
+    clearPageReveal('left', 'client-reset');
+    clearPageReveal('right', 'client-reset');
+});
 // Pagination spread updates only carry state. The playback owner decides when the
 // visible spread changes (via flips). The scene jumps directly only for non-playback
 // commits such as history restore. See docs/webgl-3d-ui-spec.md "Single ownership".