fix(snap): extension guides stay visible at extended intersections (#247, #259)

Extension osnap now offers a ray off every polyline vertex, not just the
first and last, and snaps to where two segments' extension lines would
cross. That crossing is emitted as an Intersection so it outranks the
per-segment extension feet on their own lines. When it wins, both
contributing extensions are re-detected — off-ray distance measured in
screen space so the tolerance holds at UTM coordinates — and the overlay
draws a dashed guide from each endpoint through the crossing, so both
extension paths remain visible instead of vanishing when caught.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/overlay.rs

@@ -181,6 +181,7 @@ pub fn selection_overlay<'a>(
     selection: SelectionState,
     snap: Option<(Point, SnapType)>,
     snap_ext_base: Option<Point>,
+    snap_ext_base2: Option<Point>,
     grips: Vec<GripMarker>,
     ucs_icons: Vec<UcsIconParams>,
     ost_points: Vec<OstTrackPoint>,
@@ -197,6 +198,7 @@ pub fn selection_overlay<'a>(
         selection,
         snap,
         snap_ext_base,
+        snap_ext_base2,
         grips,
         ucs_icons,
         ost_points,
@@ -220,6 +222,9 @@ struct SelectionCanvas {
     /// Screen position of the endpoint an active Extension snap extends from,
     /// so the dashed extension guide line can be drawn back to it. (#238)
     snap_ext_base: Option<Point>,
+    /// Second extension-guide base, present only for an extended intersection so
+    /// both crossing extensions stay drawn when the crossing is caught. (#247, #259)
+    snap_ext_base2: Option<Point>,
     grips: Vec<GripMarker>,
     /// One UCS icon per Model pane (each viewport shows its own at its origin);
     /// a single entry for paper / floating-viewport. Only the active pane's
@@ -706,6 +711,33 @@ impl canvas::Program<Message> for SelectionCanvas {
                     frame.stroke(&path, stroke);
                 }
                 SnapType::Intersection => {
+                    // An extended intersection sets one or two extension bases;
+                    // draw a dashed guide from each endpoint through the crossing
+                    // so both contributing extension paths stay visible. A real
+                    // on-segment crossing carries no bases, so nothing draws here.
+                    // (#247, #259)
+                    for base in [self.snap_ext_base, self.snap_ext_base2]
+                        .into_iter()
+                        .flatten()
+                    {
+                        let dx = sp.x - base.x;
+                        let dy = sp.y - base.y;
+                        let len = (dx * dx + dy * dy).sqrt();
+                        if len > 1e-3 {
+                            let dash = canvas::Stroke {
+                                line_dash: canvas::LineDash {
+                                    segments: &[4.0, 4.0],
+                                    offset: 0,
+                                },
+                                ..canvas::Stroke::default().with_color(marker).with_width(1.0)
+                            };
+                            let tip = Point::new(
+                                sp.x + dx / len * 18.0,
+                                sp.y + dy / len * 18.0,
+                            );
+                            frame.stroke(&canvas::Path::line(base, tip), dash);
+                        }
+                    }
                     let r = 5.0_f32;
                     let p1 = canvas::Path::new(|b| {
                         b.move_to(Point::new(sp.x - r, sp.y - r));