Move, turn and scale an item in the composition picture live

Dragging an object in the Composition tab's picture writes the copy
of the composition at the playhead and asks for a preview once a
frame. The tab answered every such preview by building that copy
again from the composition, which still held the old value. So the
object stood still while dragged, and a move often did nothing at
all, because its end was read from a copy that had just been rebuilt.
Values typed in the inspector were not affected.

- SceneViewport gets editing(), which says whether a gesture of this
  view is under way: a drag, a size or turn handle, a polygon corner,
  a joint anchor or a tilemap stroke.
- CompositionCanvas leaves its copy alone while such a gesture sends
  its frame previews. The command at the end writes the composition
  as before, and the copy is built again from it.

Tests:
- tsc, eslint and Prettier are clean.
- e2e 69 on melektaus-atx: ALL PASSED, 16 checks, 69l is new. An item
  dragged 60 px to the right stands at +60 px while the button is
  still down (590 to 650) and has x = 60 in the model afterwards.
- Counter-check: rebuilding the copy on every preview again turns 69l
  red. The item stays put while dragged and x is 0 after letting go.
- Gate on melektaus-atx: green, 305 files, 4714 tests.
- Turning and scaling by the handles were not tried in the browser.
  They take the same path and editing() covers them.

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, 2048 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2049 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