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/properties/resolve.ts

@@ -33,6 +33,17 @@ export function isOverridden(object: SceneObject, key: string): boolean {
   return object.overrides[key] !== undefined;
 }
 
+/**
+ * Whether the asset's own value for `key` differs from the schema default – the asset-level
+ * counterpart of an override. The inspector highlights such values and offers a reset.
+ */
+export function isModified(asset: Asset, key: string): boolean {
+  const def = schemaFor(asset.type).find((d) => d.key === key);
+  if (!def) return false;
+  const own = asset.props[key];
+  return own !== undefined && own !== def.default;
+}
+
 /** Value the object would have without its override for `key` (the asset's value). */
 export function inheritedValue(asset: Asset, key: string): PropertyValue | undefined {
   return resolveAssetProps(asset)[key];