Group timeline rows by object and edit values in the row (#408)

The rows of the animation timeline stood in the order they were keyed.
With several objects and properties the list was hard to read, and a
value could only be changed by picking a key and using the inspector.
The Play button also kept saying Pause after scrubbing had paused.

- Each object gets a header row with its name, indented by depth. The
  groups follow the scene hierarchy, the root first. Rows whose object
  is missing go to the end.
- Under an object the rows follow the order in which it lists its
  properties: X, Y, Z, then the order of the inspector. Only the view
  is sorted, the animation itself is unchanged and there is no undo
  step. After a move in the hierarchy the order is right by itself.
- The row head shows the same field as the inspector with the value at
  the playhead. Without keys it shows the value of the object, for a
  relative row the offset.
- A change in that field sets or changes the key at the playhead,
  selects it and is one undo step. A running playback pauses first.
- The Play/Pause button follows every change of the playing state, so
  scrubbing that pauses shows Play again.

Tests:
- Unit animation + timeline-view: 100 passed, 3 new.
- e2e 25 on melektaus-atx: ALL PASSED, 34 checks, 3 new (groups,
  order and values at frame 4, a typed 55 becomes a selected key on
  frame 4, Play then a click on the ruler shows Play and pauses).
- Gate on melektaus-atx: exit 0, 297 files, 4561 tests.
- tsc, eslint, Prettier clean.

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, 2004 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2007 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