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/scene/model/wire_model.rs

@@ -22,6 +22,18 @@ pub enum SnapHint {
     /// arc's two ends would also produce a midpoint snap out on the chord,
     /// which is not on the geometry.
     Endpoint,
+    /// B-rep corner of a 3D solid. Maps to the separate 3D snap system
+    /// (`SnapType::Vertex`), never to the 2D modes.
+    Vertex,
+    /// Centre of a 3D solid B-rep edge. Maps to the separate 3D snap system
+    /// (`SnapType::EdgeMidpoint`), never to the 2D modes.
+    EdgeMidpoint,
+    /// Centre of a 3D solid B-rep face. Maps to the separate 3D snap system
+    /// (`SnapType::FaceCenter`), never to the 2D modes.
+    FaceCenter,
+    /// NURBS knot location on a spline entity. Maps to the separate 3D snap
+    /// system (`SnapType::Knot`), never to the 2D modes.
+    Knot,
 }
 
 /// Geometric primitive used by the tangent-snap engine.