fix(studio): close the timeline regressions the QA triage attributed here

The QA fleet's 295 findings were replayed against the merge-base. Most were
pre-existing, but these were caused by this stack:

Deleting one keyframe destroyed the whole tween. The lane-header remove toggle
escalated a flat tween to a whole-animation delete, which took the authored
`tl.to(...)` and its source comment with it on a single click. The base build
posts remove-keyframe and lets the writer refuse it; restore that.

A keyframed layer could not be hidden at all. The visibility eye had moved off
the always-mounted layer row onto a hover-gated property-group row, so it only
existed while the lanes were expanded AND the pointer was over that lane. A
keyboard-only user could reach no eye at all, and its label named a track its
row did not act on. It goes back on the layer row.

A drag from the centre of a clip bar did nothing, because the 16px inline ease
button sits exactly there and swallowed the press. It now lets the press through
to the clip and keeps only the click, dropped if the pointer travelled.

Dragging a diamond onto a neighbour silently discarded the retime. The clamp
bounded the dragged keyframe by the whole merged row, so two animations
colliding at one percentage pinned each other in place and the drag resolved
back to a click. Clamp against the dragged keyframe's own tween instead.

Also: floor the diamond hit box at 12px (the gap-derived size fell to ~7px at
the zoom floor), round the diamond tooltip percentage, and prune the keyframe
caches when a composition switch drops a file from the scan set — each file only
ever cleared its own entries, so the previous composition leaked every element
into both keyframeCache and gsapAnimations, with nothing to evict it.

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

@@ -90,8 +90,6 @@ function renderHeader(options: RenderHeaderOptions = {}): {
           currentTime={next.currentTime ?? 0}
           isTrackHidden={false}
           isAudioTrack={false}
-          isActive
-          isHovered={false}
           theme={defaultTimelineTheme}
           onToggleClipExpanded={vi.fn()}
           onToggleTrackHidden={vi.fn()}
@@ -123,6 +121,17 @@ describe("TimelineTrackHeader", () => {
     act(() => view.root.unmount());
   });
 
+  // The eye acts on the layer, so it has to be reachable without a pointer and
+  // in every disclosure state — a hover-gated eye is unusable by keyboard.
+  it("keeps the visibility eye mounted whether the layer is expanded or collapsed", () => {
+    const view = renderHeader({ expanded: true });
+    expect(view.host.querySelector('button[aria-label="Hide track 0"]')).not.toBeNull();
+
+    view.rerender({ expanded: false });
+    expect(view.host.querySelector('button[aria-label="Hide track 0"]')).not.toBeNull();
+    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 });