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/model/Properties.ts

@@ -4,7 +4,21 @@ export type PropertyValue = number | string | boolean;
 /** Sparse map of property key → value (asset properties or scene-object overrides). */
 export type PropertyBag = Record<string, PropertyValue>;
 
+/** A point in world or screen space. */
 export interface Vec2 {
   x: number;
   y: number;
 }
+
+/**
+ * Where a scene object sits: `x`/`y` in world units, `z` is the draw order. Higher `z` is
+ * drawn on top; objects with equal `z` keep their hierarchy order.
+ */
+export interface Position extends Vec2 {
+  z: number;
+}
+
+/** Stable draw order: ascending `z`, ties keep the input (hierarchy) order. */
+export function sortByZ<T extends { z: number }>(items: readonly T[]): T[] {
+  return [...items].sort((a, b) => a.z - b.z);
+}