1.2.11: motion-path overlay (toggle in stage view-options) + type-check cleanup

Motion-path overlay
-------------------
A toggleable always-on visualization of the selected layer's
position-keyframe trajectory. Pairs with the existing kf-selection-
driven path that only appears when a keyframe is actively selected.

- src/composables/useShowMotionPaths.ts: shared ref + localStorage
  ('animatr:showMotionPaths'), default off.
- CanvasStage.vue: new motionPathOverlay computed builds dashed
  segments + a dot per keyframe for store.selectedLayerId, but only
  when the toggle is on AND no kf is explicitly selected (the kf
  view wins — it's richer with drag handles). Color: lilac
  (#a47bff) so it doesn't collide with the blue kf-selection path.
- Click a dot → seek the playhead to that keyframe's time. Cheap
  navigation aid for layers with many keyframes.
- Toggle button placed in the stage's existing zoom/view-controls
  toolbar (alongside crop / mask preview / centre crosshair / stage
  preview), not the app topbar — that's where the canvas-scoped
  view modifiers already live.

Tests (4 new under "motion-path overlay — toggle + selected-layer rendering"):
- showMotionPaths persists to localStorage.
- Toggle on with two position kfs renders the overlay; toggling
  off returns to a clean canvas.
- kf-selection path takes precedence over the toggle (mount smoke
  with both paths active simultaneously).
- The motion-paths button in .zoom-controls flips the ref.

Type-check cleanup
------------------
The 7 pre-existing TS errors that piled up over the last few patch
bumps. None affected runtime, but vue-tsc was complaining:

- src/lib/clipBounds.ts: assetFootprint read width/height directly
  off ButtonAsset, which actually has canvas.width/canvas.height.
  Split the branch and added the same handling for SvgAsset.
- src/lib/render.ts: instancesByTrack was typed via
  `typeof scene.clipInstances` (which is optional) → values were
  ClipInstance[] | undefined. Wrapped in NonNullable.
- src/lib/ruwFile.ts: BackgroundVideo has no `name` field. Synthesised
  one from page.id so the extracted file gets a readable filename
  (`scene-bg-${page.id}`).
- src/devtools/tests.ts (2 sites): polygon test fixtures missed the
  required `fill` field — added `fill: '#0f0'`.
- src/devtools/tests.ts (1 site): TS 5 widens
  `Uint8Array<ArrayBufferLike>` past BlobPart's union; cast to
  unknown as BlobPart at the new File call.

`npm run type-check` is now clean.

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

package.json

@@ -1,7 +1,7 @@
 {
   "name": "ruwio",
   "private": true,
-  "version": "1.2.10",
+  "version": "1.2.11",
   "type": "module",
   "scripts": {
     "dev": "vite",