Keep a video item in a recording when the tab goes back and forth

A recording made while switching between tabs had stretches without
the video item and without its sound, and in between the video stood
for several frames. Two things went wrong behind another tab.

- A player opened there got no first picture. It waited for
  requestVideoFrameCallback, which Chrome never calls behind a tab.
  A recording not in real time then waited 3 s per frame and took it
  without the video. Now the player takes its first picture as soon
  as it has data while the tab is hidden.
- The span of a picture was learnt from steps that skipped pictures.
  After a time behind the tab it learnt 0.133 s instead of 1/30 s,
  left out seeks within that span and never corrected it. Now it
  learns only from steps where the time sought moved at most half a
  step, so no picture can lie between them.
- Behind the tab a seek forgets which picture shows but keeps the
  span. A landing without a frame counts as the same picture only
  once the tab has been in front for 500 ms.

Found in the user's recording 7ea9146b: 5363 frames, all 1/60 s
apart, with the video missing at 10.63 s (8 frames), 43.17 s
(5 frames) and 46.93 s (28 frames), and the sound gone with it.

Tests:
- Unit tests/viewport: 377 green, 4 new or rebuilt. With the old
  rule and the first picture only by requestVideoFrameCallback both
  new tests go red.
- e2e 68 on melektaus-atx: ALL PASSED, 21 checks. New 68l: a video
  item that begins behind another tab has its colours exactly, and
  a tab switched every 350 ms gives colour changes at 0/30/60/90.
- One run in five with switches had a change one frame late, so the
  check allows one frame either way.
- Gate on melektaus-atx: green, 311 files, 4824 tests.

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, 2086 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2088 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