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>

src/editor/debug/sceneHooks.ts

@@ -51,6 +51,10 @@ export function sceneHooks({ state, editor }: HookContext) {
     sceneHeaderAt: (name: 'fit' | 'visibility') =>
       centreOf(name === 'fit' ? scenePane.fitButton : scenePane.visibilityButton),
     viewportInfo: () => viewport.debugInfo(),
+    /** The scene view's centre and zoom, and whether a zoom transition is under way (#309). */
+    sceneView: () => viewport.viewState(),
+    /** The world point under a screen point, as the scene view shows it right now. */
+    sceneWorldAt: (x: number, y: number) => viewport.worldAt(x, y),
     /** A full redraw of the Scene tab – what a pan, which only moves the camera, is compared with. */
     viewportRedraw: () => viewport.redraw(),
     /** Where a dragged asset would land in the Scene tab, and why not when it may not. */

src/editor/debug/uiHooks.ts

@@ -13,6 +13,7 @@ import {
 } from '../../ui';
 import { Theme } from '../../ui/Theme';
 import { currentFontPicker } from '../font/FontPicker';
+import { generalSettings, keepGeneralSettings } from '../generalSettings';
 import { currentGroupDialog } from '../panels/GroupNameDialog';
 import { currentSoundPicker } from '../sound/SoundPicker';
 import { rectOf, textProbe } from '../textProbe';
@@ -318,6 +319,21 @@ export function uiHooks({ app, state, editor }: HookContext) {
       const dialog = editor.projectSettingsDialog();
       return dialog ? centreOf(dialog.symbolScaleControl) : null;
     },
+    /** The "General settings" dialog (#309) as its field stands, null while shut. */
+    generalSettingsDialog: () => editor.generalSettingsDialog()?.values() ?? null,
+    /** Where to click the zoom field inside that dialog. */
+    generalSettingsZoomAt: () => {
+      const dialog = editor.generalSettingsDialog();
+      return dialog ? centreOf(dialog.zoomControl) : null;
+    },
+    /**
+     * The zoom transition in ms as kept in this browser; with `ms` it is set first. The harness sets
+     * 0 on every load, so a check reading the zoom after the wheel finds the step and not the way.
+     */
+    zoomTransition: (ms?: number) => {
+      if (ms !== undefined) keepGeneralSettings({ zoomTransitionMs: ms });
+      return generalSettings().zoomTransitionMs;
+    },
     /** The "Open project" dialog (#183): the projects it lists and the row picked. */
     projectPicker: () => editor.projectPickerDialog()?.info() ?? null,
     /** The "Snapshots" dialog (#231): its rows, newest first, the row picked and its name field. */