Record the whole editor from the main menu

The game view could already record itself through nodes. Making a
video of the editor itself, to show how something is done, needed an
outside screen recorder. The main menu now starts and stops one, with
no options to set.

- "Start recording" sits in its own group before General settings, and
  reads "Stop recording" while one runs. The name is
  "Recording <time>", the size is the whole canvas in device pixels
  (an odd row or column is cut), 60 fps always. Sound is the master,
  saving goes the same way as for the game view.
- The encoder, the sound and the Matroska writing of a take moved from
  GameRecorder.ts to RecordingTake.ts. A take now reads its pictures
  from a FrameSource, so the game view and the editor share it.
- The editor's picture is copied from the canvas after each render, in
  the same frame, so nothing is drawn twice. A resting editor does not
  render, and the recording repeats its last picture.
- The mouse pointer is drawn into the recording in the shape the editor
  shows (arrow, hand, resize arrows, I-beam, crosshair, move).
- A red dot with the running time stands beside the menu button on
  screen only. It is drawn after the copy, and the toolbar makes room
  for it.
- Drawing that dot made Pixi take it as the root for every hit test,
  and the menu would not open any more. The stage is set back as the
  last rendered object after it.

Tests:
- Gate on melektaus-atx green: 303 files, 4686 tests.
- Unit recording: 20 green, 5 new.
- e2e 67: 8 checks, 67e new. It records through the menu and decodes
  the MP4: 1280x800 at 60 fps, the pointer black at the mouse, the dot
  missing, 238 plain spots alike with the canvas. Without the pointer
  and with the dot drawn before the copy, 67e is red.
- e2e 46: the main menu lists "Start recording".

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, 2038 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2039 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