revert macro-clips button feature (keep markers + scripts)

The button authoring ended up too far from the intended model after
three correction passes. Pulling it out entirely so we can rebuild
from a clean spec. Markers, script tracks, script frames, the Action
editor, the Script DSL and the runtime for all of the above stay —
they work.

Removed:
- types: ButtonState, StateTransition, ButtonHitArea, ButtonClip,
  ButtonInstance, setButtonState/setButtonDisabled Action variants,
  Scene.buttonClips/buttonInstances
- store: activeButtonClip* getters, all add/remove/setButton* actions,
  seedDefaultButtonClip, enterButtonEdit/setActiveButtonState/
  exitButtonEdit, button-label resolution in actionRuntime
- useButtonRuntime composable, buttonBlend helper,
  ButtonInstanceDialog, sidebar Buttons section, Timeline button
  rendering + pointer handlers, CanvasStage button rendering +
  pointer state machine, exportPlayer button runtime + hit-testing
- DSL commands state/enable/disable and their parse errors
- 35 button-related tests + helpers; stale testing doc; stale design
  doc

Re-applied three uncommitted fixes that were unrelated to the button
work:
- clicking a marker no longer jumps the playhead (only drag / dblclick
  act on it)
- script tracks get a right-click context menu (rename + delete,
  color in the existing palette)
- onTracksPointerDown excludes .script-lane / .script-frame so
  clicks on empty lanes or on frame dots reach the frame-open path
  instead of being swallowed by box-select pointer capture

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

src/i18n/messages.ts

@@ -95,25 +95,6 @@ export const en = {
       message: 'The clip and all placed instances will be removed.',
       confirm: 'Delete',
     },
-    buttons: 'Buttons',
-    buttonSubtitle: 'Button · 4 states',
-    noButtons: 'No buttons yet',
-    createButtonTooltip: 'Create new button',
-    newButtonDialog: {
-      title: 'New button',
-      message: 'Button name:',
-      default: 'Button',
-      confirm: 'Create',
-    },
-    renameButtonDialog: {
-      title: 'Rename button',
-      confirm: 'Apply',
-    },
-    deleteButtonDialog: {
-      title: 'Delete button?',
-      message: 'The button and all placed instances will be removed.',
-      confirm: 'Delete',
-    },
   },
 
   properties: {
@@ -385,7 +366,6 @@ export const en = {
     moveUp: 'Move up',
     moveDown: 'Move down',
     noMarkers: 'No markers defined',
-    noButtons: 'No labeled button instances',
     type: {
       play: 'Play',
       pause: 'Pause',
@@ -393,17 +373,12 @@ export const en = {
       jumpTo: 'Jump to time',
       jumpToFrame: 'Jump to frame',
       jumpToMarker: 'Jump to marker',
-      setButtonState: 'Set button state',
-      setButtonDisabled: 'Set button disabled',
       script: 'Run script',
     },
     param: {
       time: 'Seconds',
       frame: 'Frame',
       marker: 'Marker',
-      button: 'Button',
-      state: 'State',
-      disabled: 'Disabled',
       source: 'Script source',
     },
   },
@@ -415,20 +390,6 @@ export const en = {
     remove: 'Delete frame',
     done: 'Done',
   },
-  buttonInstanceDialog: {
-    title: 'Button instance',
-    clipName: 'Button',
-    label: 'Label',
-    labelPlaceholder: 'e.g. playButton',
-    labelCollision: 'Label "{name}" is already used by another button instance.',
-    disabled: 'Start disabled',
-    onClick: 'On click',
-    transitions: 'Transitions',
-    transitionsHint: '(shared by all instances of this button)',
-    addTransition: 'Add transition',
-    remove: 'Delete instance',
-    done: 'Done',
-  },
 } as const
 
 export type Messages = typeof en