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/app/update/mod.rs

@@ -3709,6 +3709,17 @@ impl OpenCADStudio {
             Message::PspaceCommand => Task::done(Message::ExitViewport),
 
             Message::Undo => {
+                // Mid-command Ctrl+Z: a drawing command steps itself back
+                // (PLINE pops the last vertex) instead of the document undo
+                // swallowing the whole in-progress object.
+                let i = self.active_tab;
+                let step = self.tabs[i]
+                    .active_cmd
+                    .as_mut()
+                    .and_then(|c| c.on_undo_step());
+                if let Some(r) = step {
+                    return self.apply_cmd_result(r);
+                }
                 self.undo_active_tab();
                 Task::none()
             }