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/editor/inspector/fieldFactory.ts

@@ -13,8 +13,11 @@ export interface FieldRow {
   row: Widget;
   /** Push a model value into the widget (no events fired). */
   setValue(value: PropertyValue): void;
-  /** Highlight the row as overridden (scene objects) and show the reset button. */
-  setOverridden(overridden: boolean): void;
+  /**
+   * Highlight the row as deviating from its base value and show the reset button:
+   * an override on scene objects, a non-default value on assets.
+   */
+  setModified(modified: boolean): void;
 }
 
 /**
@@ -76,10 +79,10 @@ export function createFieldRow(def: PropertyDef, value: PropertyValue, handlers:
   return {
     row,
     setValue,
-    setOverridden(overridden) {
-      label.setStyle(overridden ? 'bodyBold' : 'muted');
-      if (reset && reset.visible !== overridden) {
-        reset.visible = overridden;
+    setModified(modified) {
+      label.setStyle(modified ? 'bodyBold' : 'muted');
+      if (reset && reset.visible !== modified) {
+        reset.visible = modified;
         row.relayout();
       }
     },