fix(ucs): grid axis lines and grid snap follow the active UCS

Two leftovers from the UCS grid work:
- The coloured X/Y/Z axis lines (draw_axes) were drawn at the world
  origin along world axes; they now run through the UCS origin along the
  UCS axes, matching the grid lines.
- Grid snap rounded the cursor on the world XYZ lattice; it now rounds in
  the UCS grid frame (to-UCS, round, back-to-world) so it lands on the
  grid the user sees. The UCS origin (wire space) + rotation reach the
  snapper via DocumentTab::ucs_grid_basis; identity outside model space.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/document.rs

@@ -195,6 +195,23 @@ impl DocumentTab {
         self.scene.viewcube_ucs = self.ucs_xform().rotation_mat();
     }
 
+    /// Grid origin (render/wire space) and UCS→world rotation for grid snap and
+    /// the grid overlay. Identity / origin-at-zero outside model space.
+    pub(super) fn ucs_grid_basis(&self) -> (glam::Vec3, glam::Mat4) {
+        if self.scene.current_layout != "Model" {
+            return (glam::Vec3::ZERO, glam::Mat4::IDENTITY);
+        }
+        let xf = self.ucs_xform();
+        let (o, ..) = xf.axes();
+        let wo = self.scene.world_offset;
+        let origin = glam::Vec3::new(
+            o.x - wo[0] as f32,
+            o.y - wo[1] as f32,
+            o.z - wo[2] as f32,
+        );
+        (origin, xf.rotation_mat())
+    }
+
     pub(super) fn new_drawing(n: usize) -> Self {
         let mut scene = Scene::new();
         linetypes::populate_document(&mut scene.document);

src/app/update.rs

@@ -2643,7 +2643,9 @@ impl OpenCADStudio {
                     let all_wires = self.tabs[i].scene.hit_test_wires();
                     // Grip drag has no single rubber-band origin for a perp foot.
                     self.snapper.from_point = None;
-                    let snap_hit = self.snapper.snap(raw, p, &all_wires[..], vp_mat, bounds);
+                    let (go, gr) = self.tabs[i].ucs_grid_basis();
+                    let snap_hit =
+                        self.snapper.snap(raw, p, &all_wires[..], vp_mat, bounds, go, gr);
                     let mut snapped = snap_hit.map(|s| s.world).unwrap_or(raw);
                     self.tabs[i].snap_result = snap_hit;
                     if let Some(s) = self.tabs[i].snap_result.as_mut() {
@@ -2817,9 +2819,10 @@ impl OpenCADStudio {
                             bounds,
                         )
                     } else {
+                        let (go, gr) = self.tabs[i].ucs_grid_basis();
                         self.snapper.from_point = self.last_point;
                         self.snapper
-                            .snap(cursor_world, p, &all_wires[..], view_proj, bounds)
+                            .snap(cursor_world, p, &all_wires[..], view_proj, bounds, go, gr)
                     };
 
                     // Object Snap Tracking: update dwell, then align the cursor
@@ -3445,8 +3448,9 @@ impl OpenCADStudio {
                             self.snapper
                                 .snap_tangent_only(raw, p, &all_wires[..], vp_mat, bounds)
                         } else {
+                            let (go, gr) = self.tabs[i].ucs_grid_basis();
                             self.snapper.from_point = self.last_point;
-                            self.snapper.snap(raw, p, &all_wires[..], vp_mat, bounds)
+                            self.snapper.snap(raw, p, &all_wires[..], vp_mat, bounds, go, gr)
                         };
                         // snap.world is in paper-space (projected wire coords in MSPACE);
                         // convert to model-space so commands receive consistent coordinates.