feat(ui): trackpad pan with two fingers, zoom with pinch (#386)

Two-finger scroll pans the model instead of zooming; pinch-to-zoom
zooms toward the cursor and honors reverse zoom. Trackpad scrolls
arrive bracketed (Start..End, momentum included) while mouse wheels
send bare moves, so phases tell them apart: bracketed gestures pan,
bare vertical input zooms and bare horizontal input pans (thumb wheel).
Mouse line/page input keeps zooming, and ui.scroll now sends line
input so scripts and agents are unaffected. New Navigation preference
'Two-finger scroll pans' (default on) restores the old behaviour
when off.

crates/ui-egui/src/prefs.rs

@@ -194,6 +194,8 @@ pub struct Prefs {
     pub zoom_to_cursor: bool,
     /// Spread discrete wheel input across frames (off: apply each event immediately).
     pub smooth_wheel_zoom: bool,
+    /// Two-finger trackpad scroll pans (off: it zooms like the wheel); pinch always zooms.
+    pub trackpad_pan: bool,
     /// View changes (axis view, Look At, Home) animate; `look_at_speed` scales how fast.
     pub look_at_anim: bool,
     pub look_at_speed: f32,
@@ -221,6 +223,7 @@ impl Default for Prefs {
             ui_scale: 1.0,
             zoom_to_cursor: true,
             smooth_wheel_zoom: true,
+            trackpad_pan: true,
             look_at_anim: true,
             look_at_speed: 1.0,
             up_axis: "z".into(),
@@ -322,6 +325,7 @@ fn defaults(app: &mut SolveApp, page: usize) {
             p.zoom_reverse = d.zoom_reverse;
             p.zoom_to_cursor = d.zoom_to_cursor;
             p.smooth_wheel_zoom = d.smooth_wheel_zoom;
+            p.trackpad_pan = d.trackpad_pan;
             p.orbit_cursor = d.orbit_cursor;
             p.space_mouse_enabled = d.space_mouse_enabled;
             p.space_mouse_speed = d.space_mouse_speed;
@@ -815,6 +819,11 @@ fn page(app: &mut SolveApp, ui: &mut egui::Ui, t: &Tokens) {
             ui.checkbox(&mut app.preferences.smooth_wheel_zoom, "")
                 .on_hover_text("Turn off to apply each wheel event immediately, without a smoothing tail");
             ui.end_row();
+            ui.label("Two-finger scroll pans");
+            ui.checkbox(&mut app.preferences.trackpad_pan, "").on_hover_text(
+                "Trackpad gestures pan (off: they zoom like the wheel); mouse wheel zooms, thumb wheel pans sideways, pinch always zooms",
+            );
+            ui.end_row();
             ui.label("Orbit around");
             ui.horizontal(|ui| {
                 ui.radio_value(&mut app.preferences.orbit_cursor, false, "View centre");
@@ -1119,6 +1128,26 @@ mod tests {
         assert_eq!(other.preferences.nav, OPENSCAD_NAV);
     }
 
+    #[test]
+    fn trackpad_pan_defaults_on_persists_and_follows_dialog_buttons() {
+        assert!(serde_json::from_value::<Prefs>(json!({})).unwrap().trackpad_pan);
+        assert!(Prefs::default().trackpad_pan);
+        let mut app = SolveApp::new(solvecraft_engine::Session::default(), Default::default());
+        control(&mut app, &json!({"open":"Navigation", "trackpad_pan":false})).unwrap();
+        control(&mut app, &json!({"button":"cancel"})).unwrap();
+        assert!(app.preferences.trackpad_pan);
+        control(&mut app, &json!({"open":"Navigation", "trackpad_pan":false})).unwrap();
+        control(&mut app, &json!({"button":"apply"})).unwrap();
+        assert!(!app.preferences.trackpad_pan);
+        let mut other = SolveApp::new(solvecraft_engine::Session::default(), Default::default());
+        other.load_prefs(&app.prefs());
+        assert!(!other.preferences.trackpad_pan);
+        control(&mut app, &json!({"open":"Navigation", "button":"defaults"})).unwrap();
+        assert!(app.preferences.trackpad_pan);
+        control(&mut app, &json!({"button":"cancel"})).unwrap();
+        assert!(!app.preferences.trackpad_pan);
+    }
+
     #[test]
     fn presets_map_the_mouse_and_prefs_round_trip() {
         let mut app = SolveApp::new(solvecraft_engine::Session::default(), Default::default());