refactor(studio): split the track header's lane math out of its JSX

The header file owned value sampling, readout formatting, lane-state
resolution and the JSX at once, so a formatting change and a layout change
edited the same file. Sampling and formatting now live in
trackHeaderLaneValues, lane-state resolution in trackHeaderLaneState, and
resolveLaneHeaderState returns only the four fields its caller reads.

Also shows the track's clip count next to the track identity, which the
header promised but never rendered.

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

@@ -60,6 +60,7 @@ const OPACITY = animation("opacity-tween", "visual", [
 
 interface RenderHeaderOptions {
   animations?: GsapAnimation[];
+  clipCount?: number;
   currentTime?: number;
   expanded?: boolean;
   onSeek?: (time: number) => void;
@@ -82,6 +83,7 @@ function renderHeader(options: RenderHeaderOptions = {}): {
           trackLabel="Hero card"
           contentOrigin={LABEL_COL_W}
           keyframeClip={ELEMENT}
+          clipCount={next.clipCount ?? 1}
           isExpanded={next.expanded !== false}
           animations={next.animations ?? [POSITION, OPACITY]}
           currentTime={next.currentTime ?? 0}
@@ -109,6 +111,17 @@ function click(host: HTMLElement, label: string) {
 }
 
 describe("TimelineTrackHeader", () => {
+  // The header shows one clip's lanes, so how many clips the track holds is
+  // otherwise invisible from the label column. A single-clip track stays silent.
+  it("shows the track's clip count only once the track holds more than one clip", () => {
+    const view = renderHeader({ clipCount: 1 });
+    expect(view.host.querySelector('[aria-label="1 clips"]')).toBeNull();
+
+    view.rerender({ clipCount: 3 });
+    expect(view.host.querySelector('[aria-label="3 clips"]')?.textContent).toBe("3");
+    act(() => view.root.unmount());
+  });
+
   it("adds and removes a keyframe on the explicitly targeted property-group tween", () => {
     const onTogglePropertyGroupKeyframe = vi.fn();
     const view = renderHeader({ currentTime: 0.5, onTogglePropertyGroupKeyframe });