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/core/src/audioGroups.test.ts

@@ -1,5 +1,10 @@
 import { beforeEach, describe, expect, it } from "vitest";
-import { audioGroupOf, HF_AUDIO_GROUP_ATTR, resolveAudioGroups } from "./audioGroups.js";
+import {
+  audioGroupOf,
+  HF_AUDIO_GROUP_ATTR,
+  resolveAudioGroups,
+  resolveCarveSourceIds,
+} from "./audioGroups.js";
 
 beforeEach(() => {
   document.body.innerHTML = "";
@@ -64,6 +69,47 @@ describe("audioGroupOf", () => {
   });
 });
 
+describe("resolveCarveSourceIds", () => {
+  it("expands a group id to its current members", () => {
+    document.body.innerHTML = `
+      <audio id="vo-1" data-audio-group="voiceover"></audio>
+      <audio id="vo-2" data-audio-group="voiceover"></audio>
+    `;
+    expect(resolveCarveSourceIds(document, ["voiceover"])).toEqual(["vo-1", "vo-2"]);
+  });
+
+  it("picks up a member added to the group after the carve was set (analysis-time, not frozen)", () => {
+    document.body.innerHTML = `
+      <audio id="vo-1" data-audio-group="voiceover"></audio>
+      <audio id="vo-2" data-audio-group="voiceover"></audio>
+    `;
+    expect(resolveCarveSourceIds(document, ["voiceover"])).toEqual(["vo-1", "vo-2"]);
+    document.body.insertAdjacentHTML(
+      "beforeend",
+      `<audio id="vo-3" data-audio-group="voiceover"></audio>`,
+    );
+    expect(resolveCarveSourceIds(document, ["voiceover"])).toEqual(["vo-1", "vo-2", "vo-3"]);
+  });
+
+  it("passes through a plain clip id that still exists", () => {
+    document.body.innerHTML = `<audio id="vo-1"></audio>`;
+    expect(resolveCarveSourceIds(document, ["vo-1"])).toEqual(["vo-1"]);
+  });
+
+  it("drops an id that resolves to nothing — a deleted clip, an empty or vanished group", () => {
+    document.body.innerHTML = `<audio id="vo-1"></audio>`;
+    expect(resolveCarveSourceIds(document, ["vo-1", "deleted", "no-such-group"])).toEqual(["vo-1"]);
+  });
+
+  it("dedupes and preserves first-seen order across a mix of group and plain ids", () => {
+    document.body.innerHTML = `
+      <audio id="vo-1" data-audio-group="voiceover"></audio>
+      <audio id="vo-2" data-audio-group="voiceover"></audio>
+    `;
+    expect(resolveCarveSourceIds(document, ["voiceover", "vo-1"])).toEqual(["vo-1", "vo-2"]);
+  });
+});
+
 describe(HF_AUDIO_GROUP_ATTR, () => {
   it("is the attribute name membership is keyed on", () => {
     expect(HF_AUDIO_GROUP_ATTR).toBe("data-audio-group");

packages/core/src/audioGroups.ts

@@ -51,6 +51,37 @@ export function resolveAudioGroups(root: ParentNode): HfAudioGroup[] {
   return groups;
 }
 
+/**
+ * Expand a list of source ids for a carve: a plain id passes through if it
+ * still exists, a group id expands to its CURRENT members. Resolved fresh
+ * every time — group membership is never frozen into the carve's own
+ * attribute, so adding a fourth voice to a group already named in a carve's
+ * `sources` picks it up on the next analysis without editing that carve.
+ *
+ * Dedupes and preserves first-seen order; an id that resolves to nothing
+ * (a deleted clip, an empty or vanished group) is dropped rather than kept
+ * as a dangling reference the analysis would only fail to find anyway.
+ */
+export function resolveCarveSourceIds(doc: Document, ids: readonly string[]): string[] {
+  const groupsById = new Map(resolveAudioGroups(doc).map((group) => [group.id, group] as const));
+  const seen = new Set<string>();
+  const out: string[] = [];
+  const add = (id: string): void => {
+    if (seen.has(id)) return;
+    seen.add(id);
+    out.push(id);
+  };
+  for (const id of ids) {
+    const group = groupsById.get(id);
+    if (group) {
+      group.memberIds.forEach(add);
+    } else if (doc.getElementById(id)) {
+      add(id);
+    }
+  }
+  return out;
+}
+
 /** The group a member belongs to, or null. Groups do not nest — this ignores
  * `data-audio-group` on an `<hf-audio-group>` element itself.
  *