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/settings.rs

@@ -193,7 +193,9 @@ const SNAP_ORDER: &[SnapType] = &[
 ];
 
 /// `$OSMODE` bit for each running object-snap mode.
-/// `None` for OCS-only snaps (Grid, ObjectPick) that have no standard bit.
+/// `None` for OCS-only snaps (Grid, ObjectPick) and the 3D solid snaps
+/// (Vertex, EdgeMidpoint, FaceCenter, Knot, FacePerpendicular, NearestFace),
+/// which live in the separate 3D set and have no standard bit.
 fn snap_bit(s: SnapType) -> Option<i32> {
     Some(match s {
         SnapType::Endpoint => 1,
@@ -209,7 +211,14 @@ fn snap_bit(s: SnapType) -> Option<i32> {
         SnapType::ApparentIntersection => 2048,
         SnapType::Extension => 4096,
         SnapType::Parallel => 8192,
-        SnapType::Grid | SnapType::ObjectPick => return None,
+        SnapType::Grid
+        | SnapType::ObjectPick
+        | SnapType::Vertex
+        | SnapType::EdgeMidpoint
+        | SnapType::FaceCenter
+        | SnapType::Knot
+        | SnapType::FacePerpendicular
+        | SnapType::NearestFace => return None,
     })
 }