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>

src/core/model/animation.ts

@@ -625,18 +625,27 @@ export function timelineLines(project: Project, animation: AnimationAsset): Time
     for (const track of rows) {
       const def = defs.find((d) => d.key === track.property);
       const label = def ? def.label : `${track.property} (missing)`;
-      const held = !aimed || !def ? 0 : track.relative ? 0 : heldBy(aimed, def);
-      lines.push({ kind: 'row', track, label, def, held, depth });
+      lines.push({ kind: 'row', track, label, def, held: heldBy(aimed, def, track), depth });
     }
   }
   return lines;
 }
 
-function heldBy(aimed: Aimed, def: PropertyDef): PropertyValue {
+function heldBy(
+  aimed: Aimed | undefined,
+  def: PropertyDef | undefined,
+  track: AnimationTrack,
+): PropertyValue {
+  if (!aimed || !def || track.relative) return 0;
   if (isPositionRow(aimed.asset.type, def.key)) return aimed.object?.position[def.key] ?? 0;
   return aimed.bag[def.key] ?? def.default;
 }
 
+/** What a row holds without keys (#412): the value its object has, no offset for a relative row. */
+export function trackHeld(project: Project, animation: AnimationAsset, track: AnimationTrack): PropertyValue {
+  return heldBy(aimedAt(project, animation, track.target), trackDef(project, animation, track), track);
+}
+
 /** What a row's header shows at `frame` (#408): the row's value there, else what it holds without keys. */
 export function lineValueAt(
   line: TimelineLine & { kind: 'row' },