Key a video to its end at 1x and zoom the timeline further out (#407)

To play a clip at its real speed the user had to know its length,
work out the last whole frame by hand and then scroll far to the
right to set the end key. The timeline now does the sum, and it
zooms out far enough to see a long clip in one piece.

- The context menu of a Video time row gets "Video at 1x to its end
  from frame N". It keys the playhead with the row's value there and
  the last whole frame before the clip ends with the matching time.
- The start key turns linear and keys in between are removed. The
  whole run is one undo step (KeyStraightRunCommand).
- The clip is the one the sprite wears at the playhead, also when
  the texture is keyed in the same animation (rowVideoAt).
- The entry is greyed out for relative rows, without a video, while
  the length is unknown, or when the playhead is already at the end.
- The status line names the result, e.g. "frame 572 shows 57.2 s of
  57.27 s".
- The timeline zooms out to 0.02 px per frame instead of 4 px. The
  ruler labels follow a 1-2-5 step without an upper limit.

Tests:
- Unit video-clock + timeline-view: 27 tests green, 8 new
- e2e 66 (atx, alone): ALL PASSED, 23 checks, 66l new
- Gate (atx): CHECK EXIT 0, 297 files, 4553 tests
- Counter-check: the command keeping the keys in between turns
  exactly the command test red

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, 1999 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2000 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