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/components/StudioRightPanel.tsx

@@ -1,4 +1,4 @@
-import { useCallback, useEffect, useRef } from "react";
+import { useCallback } from "react";
 import type { StudioRightPanelProps } from "./StudioRightPanel.types";
 
 export type { StudioRightPanelProps };
@@ -20,15 +20,14 @@ import { usePanelLayoutContext } from "../contexts/PanelLayoutContext";
 import { useFileManagerContext } from "../contexts/FileManagerContext";
 import { useDomEditContext } from "../contexts/DomEditContext";
 import { usePlayerStore } from "../player";
-import { waitForMediaJob } from "./studioMediaJobs";
 import {
   applyColorGradingScopeUpdate,
   EMPTY_COLOR_GRADING_SCOPE_RESULT,
   type ColorGradingScope,
 } from "./studioColorGradingScope";
-import type { BackgroundRemovalProgress } from "./editor/propertyPanelTypes";
 import { timelineKeysForSelections } from "../utils/studioHelpers";
 import { useInspectorSplitResize } from "../hooks/useInspectorSplitResize";
+import { useRemoveBackground } from "../hooks/useRemoveBackground";
 
 // fallow-ignore-next-line complexity
 export function StudioRightPanel({
@@ -45,6 +44,7 @@ export function StudioRightPanel({
   domEditSaveTimestampRef,
   recordEdit,
   onToggleElementHidden,
+  onAutoGroupCarveSources,
   onAddMediaOverlay,
 }: StudioRightPanelProps) {
   const {
@@ -163,14 +163,6 @@ export function StudioRightPanel({
     handleInspectorSplitResizeMove,
     handleInspectorSplitResizeEnd,
   } = useInspectorSplitResize();
-  const backgroundRemovalAbortRef = useRef<AbortController | null>(null);
-
-  useEffect(
-    () => () => {
-      backgroundRemovalAbortRef.current?.abort();
-    },
-    [],
-  );
 
   const renderJobs = renderQueue.jobs as RenderJob[];
   const inspectorTabActive = rightPanelTab === "design" || rightPanelTab === "layers";
@@ -237,52 +229,7 @@ export function StudioRightPanel({
     ],
   );
 
-  const handleRemoveBackground = useCallback(
-    // fallow-ignore-next-line complexity
-    async (
-      inputPath: string,
-      options: {
-        createBackgroundPlate?: boolean;
-        quality?: "fast" | "balanced" | "best";
-        onProgress?: (progress: BackgroundRemovalProgress) => void;
-      },
-    ) => {
-      const response = await fetch(
-        `/api/projects/${encodeURIComponent(projectId)}/media/remove-background`,
-        {
-          method: "POST",
-          headers: { "Content-Type": "application/json" },
-          body: JSON.stringify({
-            inputPath,
-            createBackgroundPlate: options.createBackgroundPlate === true,
-            quality: options.quality ?? "balanced",
-          }),
-        },
-      );
-      const data = (await response.json().catch(() => ({}))) as {
-        jobId?: string;
-        error?: string;
-      };
-      if (!response.ok || !data.jobId) {
-        throw new Error(data.error || `Background removal failed (${response.status})`);
-      }
-      showToast("Removing background...", "info");
-      backgroundRemovalAbortRef.current?.abort();
-      const controller = new AbortController();
-      backgroundRemovalAbortRef.current = controller;
-      try {
-        const result = await waitForMediaJob(data.jobId, options.onProgress, controller.signal);
-        await refreshFileTree();
-        showToast(`Created transparent asset: ${result.outputPath.split("/").pop()}`, "info");
-        return result;
-      } finally {
-        if (backgroundRemovalAbortRef.current === controller) {
-          backgroundRemovalAbortRef.current = null;
-        }
-      }
-    },
-    [projectId, refreshFileTree, showToast],
-  );
+  const handleRemoveBackground = useRemoveBackground(projectId, refreshFileTree, showToast);
 
   /**
    * A dial being dragged writes to the preview and stops there.
@@ -328,6 +275,7 @@ export function StudioRightPanel({
         copiedAgentPrompt={copiedAgentPrompt}
         onClearSelection={clearDomSelection}
         onToggleElementHidden={onToggleElementHidden}
+        onAutoGroupCarveSources={onAutoGroupCarveSources}
         onUngroup={handleUngroupSelection}
         onSetStyle={handleDomStyleCommit}
         onSetAttribute={handleDomAttributeCommit}