feat(snap): dashed extension guide line for the Extension osnap (#238)

The Extension object snap worked but drew no tracking path, so it was
easy to miss — users had to keep the cursor exactly on the invisible
extension line without any visual cue.

When an Extension snap is active, draw a dashed guide line from the
endpoint it extends from, through the snap point and a little beyond,
and orient the three-dot marker along that direction. SnapResult carries
the endpoint screen position (filled once the winning snap is known);
the overlay renders the guide.

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

src/app/update/viewport.rs

@@ -1068,6 +1068,7 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
                                 screen,
                                 snap_type: SnapType::ObjectPick,
                                 tangent_obj: None,
+                                extension_base: None,
                             });
                             if let Some(cmd) = self.tabs[i].active_cmd.as_mut() {
                                 cmd.set_acquisition_hint(Some(pick.label));