macro-clips slice 3: timeline rendering for scripts and button instances

- TimelineScriptTrack added to the track union; tracks computed emits one
  per ScriptTrack in trackOrder, with the show-locked filter honoring its
  locked flag
- script track row: drag handle + PhCode name badge + lock button; lane
  renders each ScriptFrame as an orange dot with optional action-count
  badge
- button instances render inside clip-track lanes as 4 cells labeled
  N/H/P/D, colored per state, with a striped pattern on disabled and a
  fixed visual width per cell
- new scrub-bar button to add a script track (PhCode icon)
- type narrowing tightened so inGroup checks skip script/group tracks
  explicitly rather than relying on the negation

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

src/i18n/messages.ts

@@ -160,6 +160,9 @@ export const en = {
     dropNewTrackHint: 'Drop a clip here to add a new track',
     addGroup: 'Add group',
     addGroupTooltip: 'Create a new track group',
+    scriptTrackName: 'Script track',
+    addScriptTrackTooltip: 'Create a new script track',
+    scriptFrameTooltip: 'Script frame — {n} action(s)',
     addKeyframeTooltip: 'Add keyframe for the selected track at the playhead',
     groupChildCount: '{n} tracks',
     groupEmpty: 'Drop tracks here',