feat(studio): group rows in the timeline, and a split disclosure

A group renders as its own row with member rows beneath it, and disclosure
splits into two independent controls: caret shows/hides a group's member
rows (structural), `∿` shows/hides any row's automation-lane rows. Plain
tracks lose their caret (nothing to disclose structurally) and keep only
`∿`. `expandedClipIds` keeps its existing keyframe-lane-state job;
`expandedGroupIds`/`expandedLaneOwnerIds` are new, independent sets.

Groups get a real position in the row/geometry pipeline rather than a
visual-only overlay: `useTimelineTrackDerivations` re-emits a group's member
tracks contiguously under a synthetic fractional anchor key
(firstMember - 0.5, the same fractional-key convention sub-composition
expansion already uses), so `rowGeometry`/keyboard-nav/virtualization treat
a group row as a first-class row without widening their key type away from
number. `TimelineLogicalRow.level` widens `1 | 2` to `1 | 2 | 3` (group /
member-under-group / lane), lanes always `owner.level + 1`.

All of it — grouped row emission, the header, the new expansion state — is
gated behind `isCanaryEnabled("audio-groups")`; disabled, `groups` resolves
empty and every new code path no-ops. `TimelineElement.audioGroup` (+
`audioGroupLabel`, resolved once per document via `resolveAudioGroups` from
B1) is parsed unconditionally, mirroring how `hidden`/`fxChain` already
flow DOM → manifest → TimelineElement — inert without the canary.

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

packages/studio/src/player/components/useTimelineMultiDragActorWindows.ts

@@ -0,0 +1,30 @@
+import { isMultiDragActive, multiDragDeltaSeconds } from "./timelineMultiDragPreview";
+import type { MultiDragPreviewInput } from "./timelineMultiDragPreview";
+import type { TimelineTimeRange } from "../lib/timelineClipIndex";
+
+/** The extra render window a live multi-drag needs: its passengers still draw
+ *  at their ORIGIN position, offset by the drag delta, outside the normal
+ *  virtualized range. Empty when nothing is dragging. */
+export function useTimelineMultiDragActorWindows(
+  multiDragPreview: MultiDragPreviewInput | null,
+  rowsVirtualized: boolean,
+  renderTimeRange: TimelineTimeRange,
+) {
+  const multiDragDelta =
+    multiDragPreview && isMultiDragActive(multiDragPreview)
+      ? multiDragDeltaSeconds(multiDragPreview)
+      : 0;
+  const actorWindows =
+    rowsVirtualized && multiDragPreview && multiDragDelta !== 0
+      ? [
+          {
+            range: {
+              start: renderTimeRange.start - multiDragDelta,
+              end: renderTimeRange.end - multiDragDelta,
+            },
+            identities: multiDragPreview.selectedKeys,
+          },
+        ]
+      : [];
+  return actorWindows;
+}