macro-clips slice 7: button state transitions

- ButtonClip transitions are now editable in ButtonInstanceDialog
  (from/to/duration/easing) and persisted on the clip via the new
  store.setButtonClipTransitions action; transitions are shared by
  every instance of the button (clip-scoped, not instance-scoped).
- lerpProps exported from interpolate.ts so callers outside the
  sampler can blend two poses.
- useButtonRuntime tracks an active transition per instance plus a
  reactive transitionTick that pokes Vue computeds each frame.
  A self-driving RAF loop runs only while some transition is active
  (idles when none), so animations work paused or playing.
- applyButtonStateChange(id, to, transitions, persistedDisabled)
  is the one entry point for state changes — it looks up a matching
  from→to transition and starts it, or flips state instantly. Pointer
  handlers in CanvasStage and store.actionRuntime.setButtonState both
  go through it.
- CanvasStage blends the two state stacks via sampleStateLayers +
  blendStates: layers matched by name interpolate their sampled props
  with lerpProps; unmatched layers fade in/out. Renderer uses the
  blended layer list in place of the pure state stack during transit.
- Export runtime mirrors the same flow: applyButtonChange starts
  transitions, sampleButtonTransition drives per-frame progress, and
  drawButtonStatesBlended composites via drawLayer(layer, 0, override).
  drawLayer gained an optional propsOverride param for this.
  RUNTIME_VERSION bumped to 6.
- Tests cover applyButtonStateChange matching logic and
  sampleTransition's time → u mapping plus done flag.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/interpolate.ts

@@ -37,7 +37,7 @@ export function lerpColor(a: string, b: string, t: number): string {
   return formatHex(lerp(pa[0], pb[0], t), lerp(pa[1], pb[1], t), lerp(pa[2], pb[2], t))
 }
 
-function lerpProps(a: AnimatableProps, b: AnimatableProps, t: number): AnimatableProps {
+export function lerpProps(a: AnimatableProps, b: AnimatableProps, t: number): AnimatableProps {
   const out: AnimatableProps = {
     x: lerp(a.x, b.x, t),
     y: lerp(a.y, b.y, t),