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>

README.md

@@ -11,8 +11,9 @@ Live: <https://nockal.com/game-maker/> · Widget playground: <https://nockal.com
 | Term | Meaning |
 |---|---|
 | **Asset** | Named thing with a type (`scene`, `camera`, `sprite`) and typed properties. Assets have **no position**. |
-| **Scene object** | An asset placed into a scene: a link to the asset + a **position** + sparse **overrides** of the asset's properties. Scenes cannot be placed. |
+| **Scene object** | An asset placed into a scene: a link to the asset + a **position** (`x`, `y`, `z`) + sparse **overrides** of the asset's properties. Scenes cannot be placed. |
 | **Override** | A per-object value that replaces the asset's value for one property. Only changed keys are stored; the inspector marks them bold and offers a reset (↺). |
+| **Default** | Every property has a schema default. Asset values that differ from it are marked bold in the inspector and can be reset (↺) the same way an override can. |
 | **Edit mode** | The viewport draws the project model; panels, grid, gizmos and drag-to-move are active. |
 | **Play mode** | A `World` is instantiated from the active scene (one entity per scene object, resolved properties as mutable state). Systems tick it; nothing writes back. **Stop discards the world.** |
 
@@ -25,8 +26,11 @@ behaviour will come with scripting in a later phase.
 
 **Editor symbols are always on top.** The viewport draws scene content (sprites) in an
 `objects` layer and editor symbols (camera icon, labels) in an `overlay` layer above it, so a
-sprite can never hide a symbol regardless of hierarchy order. Z-order between sprites is not
-configurable yet.
+sprite can never hide a symbol regardless of z or hierarchy order.
+
+**Z-order is part of the position.** `position.z` is the draw order of a scene object: higher
+`z` is drawn on top; objects with equal `z` keep their hierarchy order. It is edited in the
+inspector next to X/Y, undoable like any move, and applies in edit and play mode alike.
 
 ## Stack