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>

packages/studio/src/player/store/playerStore.ts

@@ -146,7 +146,14 @@ interface PlayerState extends KeyframeSlice, AutomationSelectionSlice, Thumbnail
     updates: Partial<
       Pick<
         TimelineElement,
-        "start" | "duration" | "track" | "zIndex" | "hasExplicitZIndex" | "playbackStart" | "hidden"
+        | "start"
+        | "duration"
+        | "track"
+        | "zIndex"
+        | "hasExplicitZIndex"
+        | "playbackStart"
+        | "hidden"
+        | "audioGroup"
       >
     >,
   ) => void;