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>

tools/e2e/25-animation.mjs

@@ -56,7 +56,13 @@ export async function animation(t) {
   );
   await evaluate("__gm.animation.rowDialogPick('an-arm', 'x')");
   await settle();
-  const armProperties = (await json('__gm.animation.rowDialog()')).properties.length;
+  const armOffered = (await json('__gm.animation.rowDialog()')).properties;
+  const armProperties = armOffered.length;
+  check(
+    'the position rows come first, Z (the draw order) after X and Y',
+    JSON.stringify(armOffered.slice(0, 3)) === JSON.stringify(['X', 'Y', 'Z']),
+    JSON.stringify(armOffered),
+  );
   const addButton = await find('Add row', true);
   await click(addButton.x, addButton.y);
   const row = (await walk()).tracks;