Z-order via position.z and reset-to-default for asset properties

- Position is now { x, y, z }; higher z draws on top, ties keep hierarchy order
- Viewport sorts the objects layer deterministically (syncDrawOrder)
- Inspector shows a Z field; asset properties that differ from the schema
  default are marked bold with a reset button (ResetAssetPropertyCommand)
- isModified() helper, unit tests, E2E checks and seed updated

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/core/serialize.test.ts

@@ -50,7 +50,7 @@ describe('serialize', () => {
       id: 'o1',
       assetId: 'sp1',
       name: '',
-      position: { x: 5, y: 0 },
+      position: { x: 5, y: 0, z: 0 },
       overrides: { color: 0x00ff00, width: 1 },
     });
     expect(project.assets[1]!.name).toBe('sprite');