feat(snap): independent 3D object-snap system with six modes

Grid snap locks to SNAPUNIT instead of magnetizing; snapped 3D points keep elevation instead of flattening to z=0. New SnapTypes Vertex/EdgeMidpoint/FaceCenter/Knot/FacePerpendicular/NearestFace with their own F4 master toggle and mode set, tiers, and markers, fully independent of the 2D master. Solid B-rep corners/edge-midpoints/face-centers from mesh edges and kernel faces, spline knot points, and live perpendicular/nearest-to-face on mesh triangles merged priority-aware in move/click paths. Drafting Settings 3D Object Snap tab is fully live (was dead placeholders); Nearest-to-face off by default. TDD: behavioral LINE-to-solid tests plus engine/scene unit tests.

src/app/update/mod.rs

@@ -26,6 +26,7 @@ fn is_modal_blocked_key_msg(msg: &Message) -> bool {
             | Message::MTextCaretMove(_)
             | Message::DeleteSelected
             | Message::ToggleSnapEnabled
+            | Message::ToggleSnap3dEnabled
             | Message::ToggleGrid
             | Message::ToggleOrtho
             | Message::ToggleGridSnap
@@ -3702,6 +3703,11 @@ impl OpenCADStudio {
                 self.persist_settings_if_changed();
                 Task::none()
             }
+            Message::ToggleSnap3dEnabled => {
+                self.snapper.toggle_snap3d();
+                self.sync_vport_display(self.active_tab);
+                Task::none()
+            }
             Message::ToggleGridSnap => {
                 self.snapper.toggle_grid_snap();
                 self.sync_vport_display(self.active_tab);
@@ -4562,6 +4568,14 @@ impl OpenCADStudio {
                 }
                 Task::none()
             }
+            Message::DraftingSettingsToggleSnapMode3d(snap_type) => {
+                if let Some(state) = &mut self.drafting_settings_state {
+                    if !state.snap3d_modes.remove(&snap_type) {
+                        state.snap3d_modes.insert(snap_type);
+                    }
+                }
+                Task::none()
+            }
             Message::DraftingSettingsSnapSelectAll => {
                 if let Some(state) = &mut self.drafting_settings_state {
                     for &(snap_type, _, _) in crate::snap::ALL_SNAP_MODES {