Let the video tab's Play start at the playhead and reset its button

Two faults in the video tab. A click into the ruler while a clip
played stopped the playback, but the button kept showing "Stop". And
Play always started at the start of the clip, wherever the playhead
stood.

- The tab now reports when the ruler stops the playback, and the
  button switches back to "Play".
- Play starts at the playhead when it lies inside the clip. Without
  a selected clip the whole video counts. Outside or at the end,
  Play starts at the beginning.
- So Stop and Play continue where the playback stopped. A clip that
  ran to its end without loop starts again at the beginning.

Tests: gate on melektaus-atx green. e2e 66 ALL PASSED, 32 checks (2
new in 66e-i). With both fixes taken back, both checks are red with
exactly the reported faults: the button shows "Stop", and Play runs
from 0.71 s, i.e. from the clip start at 0.6 s.

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, 2039 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2041 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