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/commands/sceneObjectCommands.ts

@@ -1,5 +1,5 @@
 import { isPlaceableType } from '../model/AssetTypes';
-import type { PropertyValue, Vec2 } from '../model/Properties';
+import type { Position, PropertyValue, Vec2 } from '../model/Properties';
 import { findScene, findSceneObject, type Project } from '../model/Project';
 import { createSceneObject, type SceneObject } from '../model/SceneObject';
 import { coerceValue, schemaFor } from '../properties/PropertySchema';
@@ -24,7 +24,7 @@ export class PlaceAssetCommand implements Command {
   readonly label = 'Place asset';
   readonly objectId = newId();
 
-  constructor(private readonly sceneId: string, private readonly assetId: string, private readonly position: Vec2) {}
+  constructor(private readonly sceneId: string, private readonly assetId: string, private readonly position: Vec2 | Position) {}
 
   execute(project: Project): void {
     const scene = requireScene(project, this.sceneId);
@@ -60,15 +60,16 @@ export class RemoveSceneObjectCommand implements Command {
   }
 }
 
+/** Sets the full position (x, y and draw order z) of a scene object. */
 export class MoveSceneObjectCommand implements Command {
   readonly label = 'Move object';
-  private previous: Vec2 = { x: 0, y: 0 };
+  private previous: Position = { x: 0, y: 0, z: 0 };
 
   constructor(
     private readonly sceneId: string,
     private readonly objectId: string,
-    private readonly position: Vec2,
-    previous?: Vec2,
+    private readonly position: Position,
+    previous?: Position,
   ) {
     if (previous) this.previous = { ...previous };
   }
@@ -76,7 +77,7 @@ export class MoveSceneObjectCommand implements Command {
   execute(project: Project): void {
     const object = requireObject(project, this.sceneId, this.objectId);
     this.previous = { ...object.position };
-    object.position = { x: this.position.x, y: this.position.y };
+    object.position = { x: this.position.x, y: this.position.y, z: this.position.z };
   }
 
   undo(project: Project): void {