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>

tests/core/commands.test.ts

@@ -3,6 +3,7 @@ import {
   CreateAssetCommand,
   DeleteAssetCommand,
   RenameAssetCommand,
+  ResetAssetPropertyCommand,
   SetActiveSceneCommand,
   SetAssetPropertyCommand,
 } from '../../src/core/commands/assetCommands';
@@ -54,6 +55,19 @@ describe('asset commands', () => {
   it('rejects unknown properties', () => {
     const { stack, spriteId } = setup();
     expect(() => stack.execute(new SetAssetPropertyCommand(spriteId, 'x', 5))).toThrow(/not defined/);
+    expect(() => stack.execute(new ResetAssetPropertyCommand(spriteId, 'x'))).toThrow(/not defined/);
+  });
+
+  it('resets a property to its schema default with undo', () => {
+    const { project, stack, spriteId } = setup();
+    const sprite = () => project.assets.find((a) => a.id === spriteId)!;
+    stack.execute(new SetAssetPropertyCommand(spriteId, 'height', 60));
+    stack.execute(new ResetAssetPropertyCommand(spriteId, 'height'));
+    expect(sprite().props.height).toBe(100);
+    stack.undo();
+    expect(sprite().props.height).toBe(60);
+    stack.redo();
+    expect(sprite().props.height).toBe(100);
   });
 
   it('deletes an asset together with its scene objects and restores both on undo', () => {
@@ -67,8 +81,8 @@ describe('asset commands', () => {
     stack.undo();
     expect(project.assets).toHaveLength(2);
     expect(findScene(project, sceneId)!.objects.map((o) => o.position)).toEqual([
-      { x: 1, y: 2 },
-      { x: 3, y: 4 },
+      { x: 1, y: 2, z: 0 },
+      { x: 3, y: 4, z: 0 },
     ]);
   });
 
@@ -92,7 +106,7 @@ describe('scene object commands', () => {
     const object = findScene(project, sceneId)!.objects[0]!;
     expect(object.id).toBe(place.objectId);
     expect(object.assetId).toBe(spriteId);
-    expect(object.position).toEqual({ x: 5, y: 6 });
+    expect(object.position).toEqual({ x: 5, y: 6, z: 0 });
     expect(object.overrides).toEqual({});
 
     expect(() => stack.execute(new PlaceAssetCommand(sceneId, sceneId, { x: 0, y: 0 }))).toThrow(/cannot be placed/);
@@ -105,8 +119,11 @@ describe('scene object commands', () => {
     stack.execute(place);
     const object = () => findScene(project, sceneId)!.objects.find((o) => o.id === place.objectId)!;
 
-    stack.execute(new MoveSceneObjectCommand(sceneId, place.objectId, { x: 40, y: 50 }));
-    expect(object().position).toEqual({ x: 40, y: 50 });
+    stack.execute(new MoveSceneObjectCommand(sceneId, place.objectId, { x: 40, y: 50, z: 3 }));
+    expect(object().position).toEqual({ x: 40, y: 50, z: 3 });
+    stack.undo();
+    expect(object().position).toEqual({ x: 0, y: 0, z: 0 });
+    stack.redo();
 
     stack.execute(new SetOverrideCommand(sceneId, place.objectId, 'color', 0x123456));
     expect(object().overrides).toEqual({ color: 0x123456 });
@@ -120,7 +137,7 @@ describe('scene object commands', () => {
     stack.execute(new RemoveSceneObjectCommand(sceneId, place.objectId));
     expect(findScene(project, sceneId)!.objects).toHaveLength(0);
     stack.undo();
-    expect(object().position).toEqual({ x: 40, y: 50 });
+    expect(object().position).toEqual({ x: 40, y: 50, z: 3 });
   });
 
   it('clears the redo stack on a new command', () => {

tests/core/resolve.test.ts

@@ -2,7 +2,7 @@ import { describe, expect, it } from 'vitest';
 import { buildAsset } from '../../src/core/commands/assetCommands';
 import { createSceneObject } from '../../src/core/model/SceneObject';
 import { coerceValue, defaultsFor, schemaFor } from '../../src/core/properties/PropertySchema';
-import { inheritedValue, isOverridden, resolveAssetProps, resolveSceneObjectProps } from '../../src/core/properties/resolve';
+import { inheritedValue, isModified, isOverridden, resolveAssetProps, resolveSceneObjectProps } from '../../src/core/properties/resolve';
 
 describe('PropertySchema', () => {
   it('has no position on any asset type', () => {
@@ -49,10 +49,20 @@ describe('resolve', () => {
     expect(inheritedValue(sprite, 'color')).toBe(0x00ff00);
   });
 
+  it('detects asset values that differ from the schema default', () => {
+    const sprite = buildAsset('sprite', 'Box');
+    expect(isModified(sprite, 'width')).toBe(false); // explicit default counts as unmodified
+    sprite.props.width = 250;
+    expect(isModified(sprite, 'width')).toBe(true);
+    delete sprite.props.height; // missing → resolved from default → unmodified
+    expect(isModified(sprite, 'height')).toBe(false);
+    expect(isModified(sprite, 'bogus')).toBe(false);
+  });
+
   it('keeps position on the scene object only', () => {
     const sprite = buildAsset('sprite', 'Box');
     const object = createSceneObject('o1', sprite.id, { x: 10, y: 20 });
-    expect(object.position).toEqual({ x: 10, y: 20 });
+    expect(object.position).toEqual({ x: 10, y: 20, z: 0 });
     expect('position' in sprite).toBe(false);
     expect(resolveSceneObjectProps(sprite, object)).not.toHaveProperty('x');
   });

tests/core/serialize.test.ts

@@ -50,7 +50,7 @@ describe('serialize', () => {
       id: 'o1',
       assetId: 'sp1',
       name: '',
-      position: { x: 5, y: 0 },
+      position: { x: 5, y: 0, z: 0 },
       overrides: { color: 0x00ff00, width: 1 },
     });
     expect(project.assets[1]!.name).toBe('sprite');