feat(studio,lint): carve targets voiceover groups — always, when plural

Plural voiceover carve now targets a group instead of naming each clip:
`resolveCarveSourceIds` (core `audioGroups.ts`) expands a group id to its
current members at analysis time, so a clip added to the group later is
covered without touching `sources`. The picker (`useFxCarve.ts`) offers a
grouped voice as one option instead of one row per member, tests overlap
as a union of member spans (a group overlaps the bed if ANY member does),
and prefers a qualifying group over its individual members in
`autoSourceIds`.

Picking two or more ungrouped voice clips in the carve flow now mints a
group behind them (`mintGroupId`, de-duped against every id in the
document) and writes `data-audio-group` on each picked clip atomically,
one undo entry — `createAudioGroupAndAssignMembers` in
`timelineTrackVisibility.ts` copies `setElementsHidden`'s multi-target
write shape. The DSP is untouched: `mixCarveSources` already sums
multiple sources correctly (verified in the design doc's own
investigation) — this only fixes the picker.

New lint rule `audio_carve_ungrouped_sources` (`packages/lint/src/rules/
media.ts`, alongside `audio_volume_double_automation`) warns when a
`data-fx-carve`'s `sources` names two or more plain clip ids instead of a
group — the shape that silently rots when a clip is added. `/hyperframes-
audio` states the same rule as an invariant, not a tip, with the grouped-
narration HTML example from the design doc.

The group-matching and auto-group logic (`withAutoGroupedSources`,
`collectCarveCandidates`) is split into `useFxCarveGrouping.ts` —
`useFxCarve.ts` was pushing past the 600-line cap. `resolveNextCarveSettings`
is deliberately NOT an `async function`: wrapping it in one would force a
microtask on every call, including the synchronous branch — the exact bug
`withAutoGroupedSources`'s own sync-when-possible contract exists to avoid,
and one caught via `propertyPanelAudioFxGroup.test.tsx` (10 failures)
before fixing it back to a plain function the caller conditionally awaits.

Also extracted `useEffectiveTimelineDuration` out of `App.tsx` and
`useRemoveBackground` out of `StudioRightPanel.tsx` (both pushed past 600
lines from an added prop wire), and decomposed `useFxCarve.ts`'s picker
IIFE to clear fallow's complexity gate.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

skills/hyperframes-audio/SKILL.md

@@ -236,6 +236,31 @@ so one analysis covers all of them: the bands come from all the speech there is,
 the envelopes rise wherever any of it is happening. Voices that never play while the
 bed does are left out; they cannot mask it.
 
+**A carve against more than one clip id is wrong. Group the clips and carve
+against the group.** This is an invariant, not a tip. Naming clips one by one has
+to be exhaustively right and stays right only until the next edit — a fourth
+narration clip added later plays outside the carve's awareness, and the bed
+fails to duck under it silently. Naming the group instead resolves membership at
+analysis time, so a clip added to the group later is covered without touching
+`sources` at all:
+
+```html
+<!-- group the narration, then carve the bed against the group -->
+<audio id="vo-intro" data-audio-group="voiceover" …></audio>
+<audio id="vo-middle" data-audio-group="voiceover" …></audio>
+<audio id="vo-outro" data-audio-group="voiceover" …></audio>
+
+<audio
+  id="music"
+  data-fx-carve='{"enabled":true,"sources":["voiceover"],"strength":0.25}'
+  …
+></audio>
+```
+
+A `sources` list naming two or more plain clip ids instead of a group is caught
+by the `audio_carve_ungrouped_sources` lint rule — it still works, but it is the
+version that silently rots when a clip is added.
+
 **One knob.** `strength` is 0..1 and derives everything: how deep to cut, how
 many bands, how wide, how far to favour intelligibility over raw voice energy,
 how far the level may drop, how far under the voice to aim. Those six move