Fix the incomplete proof of addition commutativity in the file plus comm.v. The file contains a partial…

Fix the incomplete proof of addition commutativity in the file plus comm.v. The file contains a partial…: a task in terminal-bench-2.0 (Harbor dataset). The theorem states that addition is commutative for natural numbers (forall n m : nat, n + m = m + n). The proof attempt uses induction but is…