Feat: UCS icon — animated XYZ axis tripod in viewport bottom-left corner

Screen-space UCS icon drawn on the existing selection canvas overlay.
Axes are projected from world space so the icon rotates with the camera,
always showing the correct X/Y/Z orientation relative to the drawing plane.
Each axis has its canonical color (X=red, Y=green, Z=blue) with arrowheads.
Icon visibility controlled by show_ucs_icon flag (default on); toggled by
UCSICON ON/OFF command which also updates paper-space viewport entities.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/commands.rs

@@ -2552,6 +2552,8 @@ impl H7CAD {
                         self.push_undo_snapshot(i, "UCSICON");
                         let visible = sub != "OFF";
                         let at_origin = sub == "ORIGIN";
+                        // Update model-space icon flag.
+                        self.show_ucs_icon = visible;
                         let mut count = 0usize;
                         for entity in self.tabs[i].scene.document.entities_mut() {
                             if let acadrust::EntityType::Viewport(vp) = entity {
@@ -2564,7 +2566,7 @@ impl H7CAD {
                         }
                         self.tabs[i].dirty = true;
                         self.command_line.push_output(&format!(
-                            "UCSICON {sub}: updated {count} viewport(s)."
+                            "UCSICON {sub}: updated {count} viewport(s) + model space."
                         ));
                     }
                     _ => {

src/app/mod.rs

@@ -47,6 +47,8 @@ pub(super) struct H7CAD {
     polar_mode: bool,
     /// Show grid lines in the viewport (F7).
     show_grid: bool,
+    /// Show the UCS icon in the bottom-left corner of model space (UCSICON).
+    show_ucs_icon: bool,
     /// Last point committed by a drawing command — used as ortho/polar base.
     last_point: Option<glam::Vec3>,
     /// OS window Id for the floating Layer Properties Manager (None when closed).
@@ -313,6 +315,7 @@ impl H7CAD {
             ortho_mode: false,
             polar_mode: false,
             show_grid: false,
+            show_ucs_icon: true,
             last_point: None,
             layer_window: None,
             main_window: None,

src/app/view.rs

@@ -58,20 +58,32 @@ impl H7CAD {
                     vec![]
                 };
 
+            let (vw, vh) = tab.scene.selection.borrow().vp_size;
+            let vp_bounds = iced::Rectangle { x: 0.0, y: 0.0, width: vw, height: vh };
+
             let grid = if self.show_grid {
-                let (vw, vh) = tab.scene.selection.borrow().vp_size;
-                let bounds = iced::Rectangle { x: 0.0, y: 0.0, width: vw, height: vh };
                 let cam = tab.scene.camera.borrow();
                 let plane = grid_plane_from_camera(cam.pitch, cam.yaw);
                 Some(overlay::GridParams {
-                    view_proj: cam.view_proj(bounds),
-                    bounds,
+                    view_proj: cam.view_proj(vp_bounds),
+                    bounds: vp_bounds,
                     plane,
                 })
             } else {
                 None
             };
-            overlay::selection_overlay(sel, snap_info, grips, grid)
+
+            let ucs_icon = if self.show_ucs_icon && !is_paper {
+                let cam = tab.scene.camera.borrow();
+                Some(overlay::UcsIconParams {
+                    view_proj: cam.view_proj(vp_bounds),
+                    bounds: vp_bounds,
+                })
+            } else {
+                None
+            };
+
+            overlay::selection_overlay(sel, snap_info, grips, grid, ucs_icon)
         };
 
         let nav = container(overlay::nav_toolbar())