refocus_tree: clear arena on no root

src/logic/future_moves.rs

@@ -535,6 +535,8 @@ impl FutureMoves {
     /// Rebuilds the Arena based on `self.current_root`, prunes unrelated nodes
     fn refocus_tree(&mut self) {
         let Some(root) = self.current_root else {
+            // handle current_root being empty (clear arena and return)
+            self.arena.clear();
             return;
         };