Keep a transition's pace through new steps and pans (#309)

A wheel step in the middle of a transition restarted the ease-out
curve at full speed, so a run of quick steps kicked at each one.
A pan called finish() first and made the zoom jump to its target.
Both made the zoom and the gliding lists feel jerky.

- A new step starts from the speed the view already has and then
  eases out. Quick steps in one direction move in one stroke. A step
  the other way still starts from rest.
- A pan takes the transition along. The zoom goes on while dragging
  and lands where it was heading, moved by the pan. This covers all
  eight pan places: scene and container, graph, plugin board,
  texture and pixel art, timeline, pattern, song and sound.
- finish() is gone, it has no callers left. Dragging the scrollbar,
  a jump to a row, another document or a new view size still end
  the transition as before.

Found: on a second step the old curve jumped to 2.47 times the
speed the view had just before.

Tests:
- npm run check on stein2: 299 files, 4598 tests (5 new)
- e2e 46 on stein2: ALL PASSED, 24 checks (1 new). A pan by 40/30
  px during a 1000 ms zoom step leaves it moving (2.33 on the way
  from 2 to 3), it lands on 3 with the world point moved by the pan.
- Counter-checks: with the pace reset to 3 the new unit test fails
  with 2.47 instead of at most 1.1. Without carry the pan test fails.
- melektaus-atx went offline mid-run, so both ran on stein2.

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, 2017 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2018 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