fix(studio): make audio-group edits transactional

[vanceingalls]

Part 6 of 12 replacing #3439. Base: wa-26e-studio-audio-state.

Why

Creating a bus and editing its attributes is one transaction across the live DOM, source files, optimistic state, undo, and sub-composition ownership.

What

• create groups in the owning composition and assign members in their own files
• resolve patch targets before optimistic writes
• mirror group volume, label, FX, automation, and hidden state after edits
• keep visibility edits, group deletion, and carve auto-grouping consistent
• prevent invalid or blocked timeline edits from partially applying

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: #3448 ← #3449 → #3450

[miga-heygen]

Review

Approve. Independent read at exact head.

Three real bug fixes in one well-scoped PR:

1. Transactional group creation: captures prior group state, restores on save failure. Emits <hf-audio-group> element (not just membership attrs) so later writes have a DOM target. Group ID validation + escapeAttr close injection.

2. Store mirroring: syncStoredGroupAttribute writes to BOTH elements and domClipChildren in one store notification — fixes the "muted group whose button stayed Mute" symptom for flat AND sub-comp members. mirroredGroupVolume correctly reads null/empty as unity (matching core), closing the Number(null)===0 parse divergence.

3. ID space boundary: useAudioGroupCarveAssignment now resolves DOM ids via runtimeAudioId, not store keys — fixes the silent match-nothing → carve-pointing-at-nothing bug. resolveGroupSourceFile climbs past the sub-comp root to the host that names the file — fixes "Unable to patch element in index.html" for sub-comp groups.

persistElementAttribute now resolves the target BEFORE the optimistic patch and reads the unwind value from the FILE, not the already-patched live DOM — closing two failure classes at once.

No issues. Ship it.

— Miga

The base branch was changed.