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/app/mod.rs

@@ -234,6 +234,8 @@ pub enum Message {
     SetPolarAngle(f32),
     /// Toggle dynamic input overlay (F12).
     ToggleDynInput,
+    /// Toggle object snap tracking (F11).
+    ToggleOTrack,
     /// Toggle an individual snap mode (from popup row click).
     ToggleSnap(crate::snap::SnapType),
     /// Open / close the OSNAP popup (▾ arrow click).