fix(studio): settle boundary retimes, delete every keyframed tween, tighten the test locks

Review follow-ups on the expanded keyframe lanes.

Writer:
- `onMoveKeyframe`'s flat-tween boundary branch answered `true` the moment it
  dispatched update-meta, so a rejected write left the diamond parked at its drop
  position. `observeGsapMutation` now resolves to whether the mutation landed and
  the boundary branch returns it, matching the other branches.
- "Delete All Keyframes" cleared only the first keyframed tween on the layer, so
  a layer with position AND opacity keyframes kept half of them. It now walks
  every keyframed tween, serially, through the clicked element's selection.
- The post-convert lookup in `commitFlatViaKeyframes` matched by target selector,
  which picks an arbitrary tween when a target carries several. Match by id first.

Interaction and a11y:
- A rejected retime whose commit settled after a newer drag reverted the
  selection to its own source keyframe, undoing a retime the user could see. The
  revert now only runs while it is still the lane's latest gesture.
- Diamonds key on the authored identity instead of index plus rendered clip-%, so
  a neighbour's retime no longer remounts the button mid-drag.
- The disclosure caret gets `aria-controls` on an always-mounted lanes container,
  and both it and the property-group toggle grow to the 24x24 WCAG 2.2 minimum.
- `LayerDisclosureRow` takes the same adaptive `columnWidth` as its sibling lane
  rows instead of hardcoding LABEL_COL_W over the canvas.

Test locks:
- The timeline callbacks harness resolves a DISTINCT selection per element, so
  the clicked-element writes are actually pinned; three assertions that passed
  either way now name the clicked element's selection.
- New: null-selection aborts every mutation, delete-all covers both tweens, a
  rejected boundary retime reports `false`, and a stale revert leaves selection.
- The playhead-percentage assertion checks 25, not `expect.any(Number)` (which
  also accepts NaN); ease segments assert their label ORDER, not just that the
  three curves differ; the collapsed-diamond callback asserts the whole target.
- Dropped a duplicate `selection override` describe left by a rebase.

packages/studio/src/player/components/LayerDisclosureRow.tsx

@@ -1,6 +1,6 @@
 import { CaretRight } from "@phosphor-icons/react";
 import type { TimelineElement } from "../store/playerStore";
-import { LABEL_COL_W, TRACK_H } from "./timelineLayout";
+import { TRACK_H } from "./timelineLayout";
 import { TrackClipCount } from "./TrackClipCount";
 
 // Layer row (Figma order: disclosure ▸/▾, diamond, name) — the disclosure lives
@@ -10,6 +10,8 @@ export function LayerDisclosureRow({
   clipCount,
   isExpanded,
   gutterBackground,
+  columnWidth,
+  lanesId,
   onToggleClipExpanded,
   children,
 }: {
@@ -17,6 +19,11 @@ export function LayerDisclosureRow({
   clipCount: number;
   isExpanded: boolean;
   gutterBackground: string;
+  /** Same adaptive width the lane rows use: a narrowed header column must not
+   *  leave this row hanging over the clips it labels. */
+  columnWidth: number;
+  /** Id of the element holding the lanes this row's caret expands. */
+  lanesId: string;
   onToggleClipExpanded: () => void;
   /** Trailing controls that act on the LAYER (the visibility eye), not on a lane. */
   children?: React.ReactNode;
@@ -26,7 +33,7 @@ export function LayerDisclosureRow({
     <div
       className="absolute left-0 top-0 flex items-center gap-1.5 overflow-hidden px-1.5 text-[11px]"
       style={{
-        width: LABEL_COL_W,
+        width: columnWidth,
         height: TRACK_H,
         color: "#ffffff",
         background: gutterBackground,
@@ -35,9 +42,12 @@ export function LayerDisclosureRow({
       <button
         type="button"
         aria-expanded={isExpanded}
+        aria-controls={lanesId}
         aria-label={`${isExpanded ? "Collapse" : "Expand"} ${name} keyframes`}
         title={`${isExpanded ? "Collapse" : "Expand"} keyframe lanes`}
-        className="flex h-5 w-4 shrink-0 items-center justify-center rounded border-0 bg-transparent p-0 text-white/55 hover:text-white focus-visible:outline focus-visible:outline-1 focus-visible:outline-[#3CE6AC]"
+        // h-6 w-6 = the 24x24 WCAG 2.2 minimum target. The caret glyph stays 11px;
+        // only the hit box grows.
+        className="flex h-6 w-6 shrink-0 items-center justify-center rounded border-0 bg-transparent p-0 text-white/55 hover:text-white focus-visible:outline focus-visible:outline-1 focus-visible:outline-[#3CE6AC]"
         onPointerDown={(event) => event.stopPropagation()}
         onClick={(event) => {
           event.stopPropagation();