feat: Object Snap Tracking (OTRACK / F11)

- snap/mod.rs: Snapper gains otrack_enabled, tracking_points, dwell state;
  update_otrack_dwell() acquires a snap point after DWELL_THRESHOLD=4
  consecutive moves near the same position; otrack_snap() returns an
  H/V alignment snap position; clear_tracking() resets state on command end
- overlay.rs: selection_overlay accepts ost_points + cursor_screen;
  draws light-blue dashed H/V lines from acquired points + small cross marker
- statusbar.rs: OTRACK pill (F11) added next to DYN
- view.rs: projects tracking_points to screen, passes to selection_overlay
- update.rs: calls update_otrack_dwell + otrack_snap in ViewportMove;
  overrides effective point with OST alignment when no normal snap is active
- ROADMAP.md: Object Snap Tracking marked ✅

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/statusbar.rs

@@ -29,6 +29,7 @@ impl StatusBar {
         polar_increment_deg: f32,
         show_grid: bool,
         dyn_input: bool,
+        otrack: bool,
         layouts: Vec<String>,
         current_layout: String,
         // If `Some((original, edit_value))`, the named tab shows a text input.
@@ -87,6 +88,10 @@ impl StatusBar {
                 toggle_pill("DYN", dyn_input, Message::ToggleDynInput),
                 "Dynamic Input\nF12"
             ),
+            tip(
+                toggle_pill("OTRACK", otrack, Message::ToggleOTrack),
+                "Object Snap Tracking\nF11"
+            ),
             osnap_btn(osnap_active, snapper.snap_enabled, popup_open),
             status_pill(space_label),
             status_pill(scale_label),