1.2.4: mask cascade strict bottom-up + clip-internal masks

Audited masking against the four-scope spec (groups, main timeline,
clips, SVG-internal). Rule 2 was already correct; rules 1, 3, 4 needed
fixes:

- drawMaskedGroup now walks children bottom-up and applies each mask
  to the offscreen buffer immediately, so a mask only affects layers
  below it in the stack — layers above the mask render unmasked on top.
- drawTrackGroup rewritten with an Item discriminated union collected
  bottom-up, then a single z-order loop renders/applies, replacing the
  old "collect all masks → apply at end" pattern.
- Both clip-rendering sites (drawSceneFrame top-level clip path and
  drawTrackGroup's clip-child path) switched from drawLayers to
  drawMaskedGroup — previously a mask layer inside a clip was rendered
  as a regular shape instead of masking.
- 5 new tests under "mask layers — rendering" cover the four-rule
  spec (group below-only, group negative below-only, clip-scoped mask,
  SVG-asset mask scoped to interior, clip-internal layers above mask
  unmasked).

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

package.json

@@ -1,7 +1,7 @@
 {
   "name": "ruwio",
   "private": true,
-  "version": "1.2.3",
+  "version": "1.2.4",
   "type": "module",
   "scripts": {
     "dev": "vite",