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/App.tsx

@@ -37,6 +37,7 @@ import { useCompositionDimensions } from "./hooks/useCompositionDimensions";
 import { useToast } from "./hooks/useToast";
 import { useCompositionContentLoader } from "./hooks/useCompositionContentLoader";
 import { useStudioUrlState } from "./hooks/useStudioUrlState";
+import { useEffectiveTimelineDuration } from "./hooks/useEffectiveTimelineDuration";
 import {
   buildStudioContextValue,
   useGlobalFileDrop,
@@ -95,13 +96,10 @@ export function StudioApp() {
   const setTimelineSelectionSet = usePlayerStore((s) => s.setSelectedElementIds);
   const timelineDuration = usePlayerStore((s) => s.duration);
   const isPlaying = usePlayerStore((s) => s.isPlaying);
-  const effectiveTimelineDuration = useMemo(() => {
-    const maxEnd =
-      timelineElements.length > 0
-        ? Math.max(...timelineElements.map((el) => el.start + el.duration))
-        : 0;
-    return Math.max(timelineDuration, maxEnd);
-  }, [timelineDuration, timelineElements]);
+  const effectiveTimelineDuration = useEffectiveTimelineDuration(
+    timelineDuration,
+    timelineElements,
+  );
   const { toasts, showToast, dismissToast } = useToast();
   const panelLayout = usePanelLayout({
     rightCollapsed: initialUrlStateRef.current.rightCollapsed,
@@ -532,6 +530,7 @@ export function StudioApp() {
                           domEditSaveTimestampRef={domEditSaveTimestampRef}
                           recordEdit={editHistory.recordEdit}
                           onToggleElementHidden={timelineEditing.handleToggleElementHidden}
+                          onAutoGroupCarveSources={timelineEditing.handleAutoGroupCarveSources}
                           onAddMediaOverlay={handleAddMediaOverlay}
                         />
                       )