Canvas · the wheel scrolls as scroll boxes do; pans hold at the press

A wheel without a modifier now slides the content by its delta, as
gpui's scroll boxes do; it moved the other way. The wheel reads the
live viewport and skips an end with no delta, and a drag pans from the
press by the pointer's whole way with the viewport held at the press,
so events that land in one frame no longer overwrite each other. Tests:
a plain wheel moves the content up with it; three wheels and an empty
end add up, and four moves pan their sum; a small frame fits at a zoom
of one.

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

src/canvas/view.rs

@@ -159,6 +159,9 @@ mod tests {
         let view = Viewport::fitting(Frame::new(0.0, 0.0, 800.0, 400.0), (440.0, 440.0), 20.0);
         assert_eq!(view.zoom, 0.5);
         assert_eq!(view.to_view((400.0, 200.0)), (220.0, 220.0));
+        let small = Viewport::fitting(Frame::new(0.0, 0.0, 100.0, 50.0), (440.0, 440.0), 20.0);
+        assert_eq!(small.zoom, 1.0, "no nearer than a zoom of one");
+        assert_eq!(small.to_view((50.0, 25.0)), (220.0, 220.0));
     }
 
     #[test]