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 <[email protected]>

src/snap.rs

@@ -205,9 +205,6 @@ impl Snapper {
         }
     }
 
-    pub fn override_active(&self) -> bool {
-        self.override_saved.is_some()
-    }
 
     /// Whether temporary tracking points are being acquired and drawn: OTRACK
     /// on, or the Extension object snap on. Extension tracks a segment's line