fix(ucs): object snap tracking rays follow the active UCS

OTRACK alignment rays (0°/90° and polar steps, plus the polar pencil from
the last point) were built along world X/Y. They now rotate with the
active UCS via a Mat4 passed into otrack_snap (scene.viewcube_ucs_mat() —
the UCS in model space, identity in paper), so tracking lines and the
intersection lock align to the user's coordinate system like ortho,
polar, and the grid. sync_ucs_to_scene now reuses UcsXform::rotation_mat.

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

src/app/update.rs

@@ -2839,12 +2839,14 @@ impl OpenCADStudio {
                             } else {
                                 None
                             };
+                            let ucs = self.tabs[i].scene.viewcube_ucs_mat();
                             self.snapper.otrack_snap(
                                 cursor_world,
                                 view_proj,
                                 bounds,
                                 step,
                                 self.last_point,
+                                ucs,
                             )
                         } else {
                             None
@@ -3464,8 +3466,9 @@ impl OpenCADStudio {
                             } else {
                                 None
                             };
+                            let ucs = self.tabs[i].scene.viewcube_ucs_mat();
                             self.snapper
-                                .otrack_snap(raw, vp_mat, bounds, step, self.last_point)
+                                .otrack_snap(raw, vp_mat, bounds, step, self.last_point, ucs)
                         } else {
                             None
                         };