fix(studio): drop dead keyframe menu wiring and close the R1 review items

The diamond context menu still declared `onChangeEase` and `onCopyProperties`
props, and `TimelineOverlays` still threaded `onChangeKeyframeEase` plus a
`keyframeCache` it never read. Nothing on any timeline branch calls them, so
they are removed along with the `onChangeKeyframeEase` callback implementation.

Also from review:
- `deleteSelectedKeyframes` only falls back to the sole keyframed animation when
  there is exactly one. A collapsed selection key carries no animation id, so
  taking the first of several deleted an arbitrary tween's keyframe.
- The duration-less retime test asserts the real 87.601% instead of
  `expect.any(Number)`, so a wrong timing basis fails it.
- `Timeline` wires the keyframe handlers' `onSelectSegment` through to the
  diamonds; it was built and then dropped, so segment ease selection never fired.
- The flat text section arms auto-focus in state rather than reading and
  clearing a ref during render, which Strict Mode's double render swallowed.

packages/studio/src/contexts/TimelineEditContext.tsx

@@ -40,7 +40,6 @@ export function TimelineEditProvider({
       value.onRazorSplitAll,
       value.onDeleteKeyframe,
       value.onDeleteAllKeyframes,
-      value.onChangeKeyframeEase,
       value.onMoveKeyframeToPlayhead,
       value.onMoveKeyframe,
       value.onToggleKeyframeAtPlayhead,