feat(ucs): central WCS↔UCS converter; adopt file UCS, drive readout + icon

Introduce one bridge between WCS (how geometry and the file are stored)
and the active UCS (the coordinate system the user works in), so every
UCS-aware system routes through it instead of re-deriving axis math.

- UcsXform (app/helpers.rs): the converter — to_wcs/to_ucs (points),
  vec_to_wcs/vec_to_ucs (directions), axes(), is_identity(). Orthonormal
  axes, so the inverse is the transpose. The old ucs_to_wcs / ucs_rotate_vec
  free fns now delegate to it (one implementation).
- DocumentTab::ucs_xform() builds it from the tab's active_ucs (None = WCS).
- DocumentTab::adopt_active_ucs_from_header() loads the document's saved
  current UCS (header model-space UCS) into active_ucs on open, so the file's
  coordinate system is live immediately; called from both load paths.
- Status-bar coordinate readout reports the cursor in the active UCS.
- UCS icon tripod projects the active UCS axes, so it rotates to the UCS
  instead of always showing world X/Y/Z.

First slice of routing the app's coordinate systems through the converter;
input/picking, grid, ViewCube, and the remaining readouts follow.

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

src/app/view.rs

@@ -154,9 +154,11 @@ impl OpenCADStudio {
 
             let ucs_icon = if self.show_ucs_icon && !is_paper {
                 let cam = tab.scene.camera.borrow();
+                let (_, ux, uy, uz) = tab.ucs_xform().axes();
                 Some(overlay::UcsIconParams {
                     view_proj: cam.view_proj(vp_bounds),
                     bounds: vp_bounds,
+                    axes: (ux, uy, uz),
                 })
             } else {
                 None
@@ -877,16 +879,19 @@ impl OpenCADStudio {
                         || tab.scene.has_selected_viewport();
                     // The cursor is tracked in local render space; re-add the
                     // model-space world offset so the readout shows true
-                    // drawing coordinates (paper space carries no offset).
+                    // drawing coordinates (paper space carries no offset), then
+                    // report it in the active UCS — the readout follows the
+                    // user's coordinate system, not raw WCS (no-op without UCS).
                     let cursor_coord = {
                         let lc = tab.last_cursor_world;
                         if is_model {
                             let wo = tab.scene.world_offset;
-                            glam::Vec3::new(
+                            let wcs = glam::Vec3::new(
                                 lc.x + wo[0] as f32,
                                 lc.y + wo[1] as f32,
                                 lc.z + wo[2] as f32,
-                            )
+                            );
+                            tab.ucs_xform().to_ucs(wcs)
                         } else {
                             lc
                         }