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>

README.md

@@ -47,7 +47,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2046 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2047 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs