feat(otrack): remove a temporary tracking point by hovering over it

There was no way to drop an acquired OTRACK point short of ending the
command. Dwelling the cursor over an existing tracking point now toggles
it off instead of re-acquiring it.

Closes #109

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/snap/mod.rs

@@ -200,15 +200,22 @@ impl Snapper {
                 if is_same {
                     self.dwell_count += 1;
                     if self.dwell_count == DWELL_THRESHOLD {
-                        // Acquire this point (max 4 tracked points).
-                        if !self.tracking_points.iter().any(|t| {
+                        // Dwelling over an already-acquired point removes it;
+                        // otherwise acquire it (max 4 tracked points).
+                        let existing = self.tracking_points.iter().position(|t| {
                             let d = (*t - p).length();
                             d < self.grid_spacing * 0.1
-                        }) {
-                            if self.tracking_points.len() >= 4 {
-                                self.tracking_points.remove(0);
+                        });
+                        match existing {
+                            Some(idx) => {
+                                self.tracking_points.remove(idx);
+                            }
+                            None => {
+                                if self.tracking_points.len() >= 4 {
+                                    self.tracking_points.remove(0);
+                                }
+                                self.tracking_points.push(p);
                             }
-                            self.tracking_points.push(p);
                         }
                     }
                 } else {