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>

ROADMAP.md

@@ -196,7 +196,7 @@ Underlay (PDF/DWF/DGN)
 | Named Views (VIEW komutu) | ✅ |
 | Named UCS kaydetme | ✅ UCS SAVE/DELETE/LIST |
 | VPORTS (viewport bölme) | ⬜ |
-| Nesne snap izleme (Object Snap Tracking) | ⬜ |
+| Nesne snap izleme (Object Snap Tracking) | ✅ F11 toggle, dwell-acquire tracking lines |
 | Dynamic Input overlay | ✅ F12 toggle, absolute XY + relative dist/angle |
 
 ---
@@ -209,7 +209,7 @@ Underlay (PDF/DWF/DGN)
 | Perpendicular / Tangent / Nearest | ✅ |
 | Grid snap | ✅ Zoom-adaptive spacing |
 | Polar tracking | ✅ Configurable angle, guide line |
-| Object snap tracking | ⬜ |
+| Object snap tracking | ✅ Dwell-acquire, H/V tracking lines, alignment snap |
 
 ---