fix(studio): keep preview state synchronized

[vanceingalls]

Part 7 of 12 replacing #3439. Base: wa-26f-studio-group-editing.

Why

Preview hydration and DOM edit sessions must agree on project/session identity, expanded children, and message ordering before the timeline UI consumes them.

What

• decompose timeline sync hydration and callback routing
• preserve audio-group state for expanded sub-compositions
• keep DOM edit sessions stable across HMR and provider remounts
• route visibility, attribute, delete-member, and playback updates through one sync path
• remove stale group-level message handling while retaining its compatibility module until PR 11

Verification

• all relevant TypeScript projects pass cumulatively
• fallow audit --base main --fail-on-issues

The final stack tip preserves the verified #3439 replacement and includes the review fixes landed across the stack.

Stack: #3449 ← #3450 → #3451

[miga-heygen]

Review

Approve. Independent read at exact head.

Two structural improvements and several bug fixes:

HMR-stable contexts: createStableContext keys contexts on globalThis by name, so a Vite HMR re-evaluation reuses the original object. Collision detection warns loudly when two modules share a name. This fixes the "useX must be used within XProvider" crashes that fired on edits to files nowhere near the context — anything that propagated an HMR boundary was enough.

Timeline sync hydration extraction: useTimelineSyncCallbacks.ts was 642 lines; the pure functions (clipTreeParentMap, collectSubCompositionDomChildren, buildTimelineElementsFromClips, withImplicitDomLayers, seekAdapterToRestorePoint, hydrateTimelineFromPreview) now live in timelineSyncHydration.ts, each callable and testable on its own.

Bug fixes: sub-comp DOM children now carry audio-group state via readChildAudioGroupState; useEffectiveTimelineDuration delegates to the guarded shared function (fixes NaN propagation); sub-comp child track fractions confined to 0.5 * (k+1) / (n+1) (strictly under 0.5, avoiding the group anchor collision at exactly 0.5 for two-child hosts); handleGroupSelection refuses audio clips in layout groups.

Import cycle break (useLivePlayheadTime → playerStore directly) and group-levels message removal are clean.

No issues. Ship it.

— Miga

The base branch was changed.