Key what an empty timeline row shows on a double-click (#412)

A double-click into a timeline row without keys set the property's
default, while the field in the row header showed the value the target
actually holds. A sprite standing on x 40 got a key with 0, so the
first key moved it. The double-click now keys the value the row shows.

- trackHeld in src/core/model/animation.ts says what a row without keys
  holds. The field in the row header and SetAnimationKeyCommand both
  ask it, so they cannot drift apart again.
- A relative row still gets 0, a row with keys still takes the value of
  its curve at that frame, and a row whose target is gone still only
  gives a notice and no key.
- README and the wiki page Development name 2047 checks.

Tests:
- Unit animation: 87 green, 1 new. A sprite on x 40 with the override
  scale 2 gets keys with 40 and 2. Red with the old def.default.
- e2e 25 on melektaus-atx: ALL PASSED, 35 checks, 25i-ii new. The arm
  stands on x 40 and a double-click on frame 3 keys { frame: 3,
  value: 40 }.
- Gate on melektaus-atx: green, 304 files, 4703 tests.

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

tools/e2e/25-animation.mjs

@@ -478,6 +478,24 @@ export async function animation(t) {
     JSON.stringify({ whilePlaying, scrubbed }),
   );
 
+  // 25i-ii. A double click into a row without keys keys what its header showed (#412): the Arm
+  //         stands at x 40, not at the property's default 0.
+  await mutate(`
+    const p = __gm.state.project;
+    p.assets.find((a) => a.id === 'an-box').objects[0].position.x = 40;
+    p.assets.find((a) => a.id === 'an-walk').tracks = [{ id: 'an-r4', target: 'an-arm', property: 'x', keys: [] }];
+  `);
+  await sleep(150);
+  const shownX = (await json('__gm.animation.lines()')).find((l) => l.kind !== 'group')?.value;
+  const emptyCell = await json("__gm.animation.cellAt('an-r4', 3)");
+  await doubleClick(emptyCell.x, emptyCell.y);
+  const keyedX = (await walk()).tracks[0].keys;
+  check(
+    'a double click into an empty row keys the value its header showed – 40, not the default 0',
+    shownX === 40 && JSON.stringify(keyedX) === JSON.stringify([{ frame: 3, value: 40 }]),
+    JSON.stringify({ shownX, keyedX }),
+  );
+
   await mutate(`
     const p = __gm.state.project;
     p.assets = p.assets.filter((a) => !['an-walk', 'an-box', 'an-sprite'].includes(a.id));