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>

src/core/store/serialize.ts

@@ -67,7 +67,7 @@ function normalizeObjects(raw: unknown, types: Map<string, AssetType>): SceneObj
       id: o.id,
       assetId: o.assetId,
       name: typeof o.name === 'string' ? o.name : '',
-      position: { x: toNumber(p.x), y: toNumber(p.y) },
+      position: { x: toNumber(p.x), y: toNumber(p.y), z: toNumber(p.z) },
       overrides: normalizeBag(o.overrides, type, true),
     });
   }