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/plugins/NodeBoard.ts

@@ -2,7 +2,14 @@ import { Container, Graphics } from 'pixi.js';
 import type { NodeDef } from '../../core/graph';
 import { Button, Label, type Size, Spacer, Theme, VStack, Widget } from '../../ui';
 import { type Box, frameBoxes } from '../framing';
-import { drawGraphGrid, FIT_MARGIN, type GraphView, panAndZoom, ZOOM_LIMITS } from '../graph/graphView';
+import {
+  drawGraphGrid,
+  FIT_MARGIN,
+  type GraphView,
+  graphViewTween,
+  panAndZoom,
+  ZOOM_LIMITS,
+} from '../graph/graphView';
 import { NodeView } from '../graph/NodeView';
 import { TabHeader } from '../TabHeader';
 
@@ -53,6 +60,11 @@ class BoardCanvas extends Widget {
   private view: GraphView = { pan: { x: 0, y: 0 }, zoom: 1 };
   /** Framed once there is a size and something to frame; after that the view is the user's. */
   private needsFit = true;
+  private readonly tween = graphViewTween(
+    () => this.view,
+    (view) => (this.view = view),
+    () => this.applyView(),
+  );
 
   constructor() {
     super();
@@ -66,6 +78,8 @@ class BoardCanvas extends Widget {
         this.view = view;
         this.applyView();
       },
+      ZOOM_LIMITS,
+      this.tween,
     );
   }
 
@@ -84,6 +98,11 @@ class BoardCanvas extends Widget {
     else this.applyView();
   }
 
+  /** Fit by hand: the Fit button and a plugin's call, shown as a transition. */
+  refit(): void {
+    this.tween.change(() => this.fit());
+  }
+
   fit(): void {
     if (this.w === 0 || this.h === 0) return;
     const boxes: Box[] = this.views.map((v) => ({ id: v.node.id, x: v.x, y: v.y, w: v.w, h: v.h }));
@@ -151,7 +170,7 @@ export class NodeBoard extends VStack {
   }
 
   fit(): void {
-    this.canvas.fit();
+    this.canvas.refit();
   }
 
   /** The nodes as drawn and the view – what the check scripts read. */