macro-clips slice 4d: button canvas runtime + pointer handling

- ButtonInstance gains optional x/y; new instances default to stage
  center and setButtonInstancePosition updates them
- useButtonRuntime composable holds per-instance state/disabled
  overrides (runtime-only, not persisted). getState layers the
  runtime disabled override on top of the persisted initial flag.
- store.actionRuntime's setButtonState/setButtonDisabled now drive
  useButtonRuntime instead of mutating the scene, so script-triggered
  state changes stay ephemeral
- CanvasStage renders each visible (playhead >= startTime) button
  instance inside a <v-group> at (bi.x, bi.y), drawing the current
  state's layer stack with draggable/listening disabled plus a
  transparent hit-rect sized to the clip's hitArea or the bbox of
  the first rect/circle layer. The group wires mouseenter/leave/
  down/up into the state machine and fires onClick via evalAction.
- exportPlayer runtime mirrors the same thing: getEffectiveButtonState,
  per-frame buttonHits array, canvas-level mousemove/down/up handlers
  do their own hit testing in scene space. RUNTIME_VERSION bumped to 5.
- tests cover useButtonRuntime defaults/overrides and the
  store.actionRuntime → runtime-state roundtrip via label resolution.

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

src/stores/scene.ts

@@ -27,6 +27,7 @@ import {
   scriptFramesInRange,
   type ActionRuntime,
 } from '@/lib/actions'
+import { useButtonRuntime } from '@/composables/useButtonRuntime'
 
 const uid = () => Math.random().toString(36).slice(2, 10)
 
@@ -1215,10 +1216,18 @@ export const useSceneStore = defineStore('scene', {
         buttonClipId,
         clipTrackId,
         startTime: Math.max(0, Math.min(this.scene.duration, startTime)),
+        x: this.scene.canvas.width / 2,
+        y: this.scene.canvas.height / 2,
       }
       ;(this.scene.buttonInstances ||= []).push(inst)
       return inst
     },
+    setButtonInstancePosition(id: string, x: number, y: number) {
+      const target = this.scene.buttonInstances?.find((bi) => bi.id === id)
+      if (!target) return
+      target.x = x
+      target.y = y
+    },
     removeButtonInstance(id: string) {
       if (!this.scene.buttonInstances) return
       this.scene.buttonInstances = this.scene.buttonInstances.filter((bi) => bi.id !== id)
@@ -1545,6 +1554,7 @@ export const useSceneStore = defineStore('scene', {
         if (byId.has(ref)) return ref
         return byLabel.get(ref) ?? null
       }
+      const btnRt = useButtonRuntime()
       return {
         fps: self.scene.fps,
         playbackIn: self.playbackIn,
@@ -1560,18 +1570,23 @@ export const useSceneStore = defineStore('scene', {
         pause() {
           self.pause()
         },
-        setButtonState() {
-          // No-op in editor for now — runtime-state management lands in
-          // the export-runtime slice together with pointer handling.
-        },
-        setButtonDisabled(ref: string, disabled: boolean) {
+        setButtonState(ref, state) {
           const id = resolve(ref)
           if (!id) {
             // eslint-disable-next-line no-console
             console.warn('[animatr] unknown button ref:', ref)
             return
           }
-          self.setButtonInstanceDisabled(id, disabled)
+          btnRt.setState(id, state)
+        },
+        setButtonDisabled(ref, disabled) {
+          const id = resolve(ref)
+          if (!id) {
+            // eslint-disable-next-line no-console
+            console.warn('[animatr] unknown button ref:', ref)
+            return
+          }
+          btnRt.setDisabled(id, disabled)
         },
         warn(msg: string) {
           // eslint-disable-next-line no-console