b12006: memory: refuse rollbacks past the snapshots of the last ubatch (#31368)

memory: refuse rollbacks past the snapshots of the last ubatch The recurrent memory and the DSV4 cache accepted any rollback up to n_rs_seq, whether or not the last ubatch wrote that snapshot. Each now records the deepest rollback its last ubatch guarantees and seq_rm refuses anything past it: recurrent: n - 1 for a ubatch of n <= n_rs_seq + 1 tokens, 0 for a longer one, which only guarantees the final state DSV4: min(n, n_rs_seq), the snapshots are rebuilt from the state before the ubatch The dead rollback block of the rollback test becomes the assertion it asked for, and rollback_bound checks the exact bound of each memory after ubatches of 1, 2, n_rs_seq + 1 and n_rs_seq + 2 tokens. tests: address review from ggerganov Detect architectures with llama_arch_from_string in comp_padding and arch_reserves_single_seq, as in rollback_bound.