Point cloud move gizmo

A lone unlocked point cloud selected in a 3D visual style shows a move
gizmo at its insertion point: X/Y/Z arrows and XY/YZ/ZX squares. Dragging
an arrow moves the cloud along that axis, a square moves it in that
plane; the hovered part is highlighted and each drag is one undo step.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/model/object.rs

@@ -135,6 +135,11 @@ pub enum GripShape {
     Dropdown,
     /// Menu selector placed immediately beside its anchor grip.
     DropdownAdjacent,
+    /// Move-gizmo arrow along world X (0), Y (1) or Z (2); the grip's world
+    /// point is the gizmo centre, the marker sits at the arrow tip.
+    GizmoAxis(u8),
+    /// Move-gizmo square between two axes: XY (0), YZ (1) or ZX (2).
+    GizmoPlane(u8),
 }
 
 /// Describes one grip point for an entity.