Zoom the video tab's time smoothly like the other tabs (#377)

The video tab jumped on every wheel step and on Fit. The Sound,
Pattern and Song tabs already ease the zoom since #377. The video
tab now takes the same transition, so all tabs feel alike.

- VideoCanvas runs wheel and Fit through `axisTween`. The duration
  comes from the General settings. Quick wheel steps add up, panning
  ends the transition, and the view that is stored is the one the
  transition lands on.
- Opening a video still fits at once without a transition. Only the
  Fit button eases, as in the Sound tab.
- `__gm.videoTab.zoom()` reports the zoom and whether a transition is
  on its way.

Tests:
- e2e 66 on melektaus-atx: ALL PASSED, 30 checks (1 new). The new
  check uses `sampleZoom` like the Sound tab. At 1000 ms the zoom is
  on its way two frames after the wheel (319 -> 321) and lands where
  the jump does (370.63). No counter-check against the old code.
- Gate on melektaus-atx: CHECK EXIT 0, 299 files, 4586 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, 2016 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2017 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