Let an animation key Z so an object can pass behind another

The timeline offered X and Y as position rows but never Z. The list
of position rows was simply written without it when animations came
in with #97. A swinging weapon could therefore not go behind its
character.

- Z is the third position row, after X and Y, on every placed root
  or object. A scene and a behavior still offer no position.
- Play and preview already read and write any position row by name,
  so the one entry in POSITION_KEYS is all the code change.
- The draw order is sorted on every render, and play writes z back
  into the scene each frame. A keyed Z therefore takes effect at
  once. A blended 4.5 sits above 4 and below 5.

Tests:
- ./check.sh: 165 files, 2416 tests, CHECK EXIT 0.
- 3 new unit tests in tests/core/animation.test.ts: Z is offered
  after X and Y and not on a behavior or scene, a running game moves
  the object from behind to in front of another one, and play,
  scene write-back and preview agree on z.
- e2e 25: ALL PASSED, 29 checks. The new check reads X, Y, Z as the
  first rows of the row dialog. Check sum 1396 -> 1397.
- Counter-check: without z in the list all 3 new unit tests fail.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/model/animation.ts

@@ -25,8 +25,11 @@ export type Playback = (typeof PLAYBACKS)[number];
 /** What an animation can be built over: something placed, or a whole scene. */
 export const ANIMATION_ROOT_TYPES: readonly AssetType[] = ['sprite', 'text', 'container', 'scene'];
 
-/** Position lives on the object, not in its property bag, so the timeline offers it by name. */
-const POSITION_KEYS = ['x', 'y'] as const;
+/**
+ * Position lives on the object, not in its property bag, so the timeline offers it by name. `z` is
+ * the draw order, which is sorted on every render, so a keyed `z` moves the object in front or behind.
+ */
+const POSITION_KEYS = ['x', 'y', 'z'] as const;
 
 function isPositionKey(key: string): key is (typeof POSITION_KEYS)[number] {
   return (POSITION_KEYS as readonly string[]).includes(key);