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/helpers.rs

@@ -118,6 +118,17 @@ impl UcsXform {
     pub(super) fn axes(&self) -> (glam::Vec3, glam::Vec3, glam::Vec3, glam::Vec3) {
         (self.origin, self.x, self.y, self.z)
     }
+
+    /// UCS→world rotation matrix (columns = UCS axes). For consumers that take
+    /// a `Mat4` rotation directly (ViewCube, OTRACK ray directions).
+    pub(super) fn rotation_mat(&self) -> glam::Mat4 {
+        glam::Mat4::from_cols(
+            self.x.extend(0.0),
+            self.y.extend(0.0),
+            self.z.extend(0.0),
+            glam::Vec4::W,
+        )
+    }
 }
 
 // ── UCS ↔ WCS transforms (thin wrappers over `UcsXform`) ────────────────────