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/ui/icons.rs

@@ -627,6 +627,14 @@ pub fn osnap(snap: crate::snap::SnapType) -> &'static [u8] {
         S::Grid => OSNAP_GRID,
         // Not shown in the snap menu; fall back to a neutral marker.
         S::ObjectPick => OSNAP_NEAREST,
+        // 3D modes have no dedicated menu glyphs yet; reuse the closest 2D
+        // shapes (hollow diamond ≈ vertex, hollow triangle ≈ edge midpoint).
+        S::Vertex => OSNAP_NODE,
+        S::EdgeMidpoint => OSNAP_MIDPOINT,
+        S::FaceCenter => OSNAP_CENTER,
+        S::Knot => OSNAP_NODE,
+        S::FacePerpendicular => OSNAP_PERPENDICULAR,
+        S::NearestFace => OSNAP_NEAREST,
     }
 }