fix(studio): scope lane ids per timeline and stop inventing a track row

Two latent defects in the announcement path this branch adds.

- lanesId was keyed by render row alone, so a second TimelineLanes on the
  page (a mini-timeline beside the main one) would mint the same
  timeline-lanes-track-0 and every caret's aria-controls would resolve to
  whichever instance mounted first. The prefix now comes from useId, with the
  colons stripped so the id stays a legal CSS selector.
- trackDisplayNumber returned trackOrder.length + 1 for a key it could not
  find, which is indistinguishable from a real row: the label announced a row
  the user can see is wrong and nothing upstream could tell it had guessed. It
  returns null now, and trackDisplaySuffix drops the number from the label
  rather than inventing one.

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

@@ -15,6 +15,7 @@ import {
   type TimelinePropertyLane,
 } from "./trackHeaderLaneState";
 import { valueReadout } from "./trackHeaderLaneValues";
+import { trackDisplaySuffix } from "./timelineTrackDisplay";
 
 interface TimelineTrackHeaderProps {
   /** The track's real key: a FRACTIONAL z-order sort value. Routes callbacks;
@@ -22,8 +23,9 @@ interface TimelineTrackHeaderProps {
   trackNumber: number;
   /** The track's 1-based position in the rendered order: the only number safe
    *  to put in a label. Announcing `trackNumber` read out "track
-   *  0.16666666666666666". */
-  trackDisplayNumber: number;
+   *  0.16666666666666666". Null when the key has no row, which drops the number
+   *  from the label rather than inventing one (see trackDisplayNumber). */
+  trackDisplayNumber: number | null;
   trackLabel: string;
   /** Id of the canvas-side lanes element the disclosure caret expands. Minted by
    *  TimelineLanes, which is the one place that sees both subtrees. */
@@ -55,14 +57,15 @@ function VisibilityButton({
 }: {
   hidden: boolean;
   trackNumber: number;
-  trackDisplayNumber: number;
+  trackDisplayNumber: number | null;
   visible: boolean;
   onToggle: TimelineEditCallbacks["onToggleTrackHidden"];
 }) {
   if (!visible) return <span aria-hidden="true" className="h-6 w-6 shrink-0" />;
   // Display number in the text, real key in the callback. The two must not be
   // conflated in either direction.
-  const label = hidden ? `Show track ${trackDisplayNumber}` : `Hide track ${trackDisplayNumber}`;
+  const suffix = trackDisplaySuffix(trackDisplayNumber);
+  const label = hidden ? `Show track${suffix}` : `Hide track${suffix}`;
   return (
     <button
       type="button"