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/mod.rs

@@ -2580,6 +2580,8 @@ pub enum Message {
     CommandEscape,
     /// Toggle the global snap on/off (OSNAP button body click).
     ToggleSnapEnabled,
+    /// Toggle 3D object snap on/off — F4.
+    ToggleSnap3dEnabled,
     /// Toggle grid-snap on/off — F9 / SNAP status-bar button.
     ToggleGridSnap,
     /// Enable or disable isometric drafting.
@@ -2752,6 +2754,7 @@ pub enum Message {
     DraftingSettingsToggleOsnap,
     DraftingSettingsToggleOtrack,
     DraftingSettingsToggleSnapMode(crate::snap::SnapType),
+    DraftingSettingsToggleSnapMode3d(crate::snap::SnapType),
     DraftingSettingsSnapSelectAll,
     DraftingSettingsSnapClearAll,
     DraftingSettingsToggle3dOsnap,