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/shortcuts.rs

@@ -17,6 +17,7 @@ pub(super) fn default_bindings() -> BTreeMap<String, String> {
         ("F1".to_string(), "HELP"),
         ("F2".to_string(), "COMMANDHISTORY"),
         ("F3".to_string(), "TOGGLEOSNAP"),
+        ("F4".to_string(), "TOGGLE3DOSNAP"),
         ("F5".to_string(), "ISOPLANE"),
         ("F7".to_string(), "GRID"),
         ("F8".to_string(), "ORTHO"),
@@ -159,6 +160,7 @@ pub(super) const INPUT_ACTIONS: &[&str] = &[
     "CARETRIGHT",
     "COMMANDHISTORY",
     "TOGGLEOSNAP",
+    "TOGGLE3DOSNAP",
     "OTRACK",
     "DYNINPUT",
     "SELECTALL",
@@ -314,6 +316,7 @@ impl OpenCADStudio {
             "CARETRIGHT" => Message::MTextCaretMove(1),
             "COMMANDHISTORY" => Message::CommandHistoryToggle,
             "TOGGLEOSNAP" => Message::ToggleSnapEnabled,
+            "TOGGLE3DOSNAP" => Message::ToggleSnap3dEnabled,
             "OTRACK" => Message::ToggleOTrack,
             "DYNINPUT" => Message::ToggleDynInput,
             "SELECTALL" => Message::SelectAllShortcut,