Show a wheel step or Fit as a short zoom transition (#309)

The wheel and the Fit button used to jump to the new view. Now the
view takes a short way there, 200 ms by default, everywhere the editor
zooms. The zoom steps stay the same, only the change is shown.

- viewTween: tweenView is the way between two views as a pure
  function. The scale moves by equal ratios, and the world point that
  sits at the same place on screen in both views stays there. After a
  wheel step that is the point under the pointer.
- ViewTween drives it on the ticker. A surface hands it reading,
  writing and drawing its own view and wraps its wheel and Fit code in
  change(). A step during a transition starts from where that one was
  heading, so quick steps add up to exactly as many steps.
- Anything else that moves the view ends the transition. A pan jumps
  to the target first, and saving a view saves the target.
- Wired into FreeViewport (Scene, Container, animation preview),
  SheetCanvas (Texture, Curve, Pixel art, Vector art), GraphCanvas,
  panAndZoom (Tile board, plugin node board) and the timeline.
- Menu > General settings... sets the time in ms, 0 jumps at once. It
  is kept in this browser like the panel sizes, not in the project,
  and it is not an undo step.
- A document that opens still comes up framed at once. The animation
  preview frames a new subject itself instead of calling fit().
- The pixel art readout names the step the view is heading for.

Measured in the scene view at 200 ms on melektaus-atx, where every
frame of the transition is a full redraw:

- 500 sprites: redraw 6.7 ms, 16 frames, mean 18 ms
- 1000 sprites: redraw 13.0 ms, 9 frames, mean 27 ms
- 2000 sprites: redraw 32.2 ms, 5 frames, mean 58 ms

Tests:
- The harness sets 0 ms on every load, so the existing modules read
  the step and not the way.
- e2e 46i samples one wheel step at 1000 ms a frame later: zoom 1.033
  between 1 and 2, the point under the pointer kept at (134, -40). It
  lands on the view a 0 ms jump reaches.
- e2e 46 and 49 expect the new menu entry.
- Gate (atx): 234 files, 3563 tests, CHECK EXIT 0 (+15)
- e2e 46,49 (atx): ALL PASSED, 43 checks (+3, sum 1735)
- Three counter-checks fail view-tween.test.ts as expected. e2e 46i
  fails when the scene view does not carry its centre along.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/lib/harness.mjs

@@ -488,6 +488,8 @@ function makeTools({ cdp, projectId, baseUrl, consoleLog, check, shotPrefix }) {
     // `state` rather than the bare hook: the sign-in screen installs one of its own, and a run
     // that stopped there must not look like a loaded editor.
     check('debug hook present', Boolean(await evaluate('Boolean(window.__gm?.state)')));
+    // The zoom is shown as a transition (#309); a check reads the step it lands on, not the way.
+    await evaluate('__gm.zoomTransition?.(0)');
   };
 
   return {