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/Entity.ts

@@ -15,5 +15,7 @@ export interface Entity {
   name: string;
   x: number;
   y: number;
+  /** Draw order; higher is drawn on top. */
+  z: number;
   props: PropertyBag;
 }