fix(undo): grip-drag history entry + mid-command Ctrl+Z steps (#332)

Grip drags mutated the document without ever pushing history — dragging
a polyline vertex (or any grip) could not be undone. The release-commit
now swaps the pre-drag backup in, snapshots that PRE state as a "GRIP"
entry, swaps the dragged result back and clears the redo stack; the
Esc-cancel path is unchanged and a no-move engaging click records
nothing.

Ctrl+Z during a drawing command steps the command instead of the
document undo swallowing the in-progress object (which also killed the
active command): new CadCommand::on_undo_step hook, consulted before
undo_active_tab. PLINE pops its last vertex (shared with the U option),
LINE reverts the last committed segment and keeps drawing from the
previous point (rides its existing U/UndoDocument path), and ARC
Continue reverts the last arc and resumes from the prior anchor with
the correct tangent via a new anchor history. With nothing placed yet
each falls through to the document undo.

Also drops the two warnings the #336 work left behind (unused Entity
import, dead Snapper::override_active).

Closes #332

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/command.rs

@@ -1218,6 +1218,14 @@ pub trait CadCommand: Send {
     /// during TRIM and trims during EXTEND, #336). Default no-op.
     fn set_shift(&mut self, _shift: bool) {}
 
+    /// Mid-command Ctrl+Z: a multi-point drawing command can take the undo
+    /// itself (PLINE pops its last vertex) instead of the document undo
+    /// swallowing the whole in-progress object. `None` (default) lets the
+    /// normal document undo run.
+    fn on_undo_step(&mut self) -> Option<CmdResult> {
+        None
+    }
+
     /// Called when the user left-clicks in the viewport (point pick).
     fn on_point(&mut self, pt: DVec3) -> CmdResult;