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/app/update/viewport.rs

@@ -1040,6 +1040,7 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
                                 snap_type: SnapType::ObjectPick,
                                 tangent_obj: None,
                                 extension_base: None,
+                                extension_base2: None,
                             });
                             if let Some(cmd) = self.tabs[i].active_cmd.as_mut() {
                                 cmd.set_acquisition_hint(Some(pick.label));