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/runtime/World.ts

@@ -34,6 +34,7 @@ export class World {
         name: object.name || asset.name,
         x: object.position.x,
         y: object.position.y,
+        z: object.position.z,
         props: resolveSceneObjectProps(asset, object),
       });
     }