Checkpoint and WAL Replay

Given snapshot (string-to-integer dictionary), checkpoint (nonnegative sequence number), and wal records, recover state. Each record is [sequence, operation, key, value], with sequence >= 1 and operation 'set' or 'delete' (value ignored for delete). Process records in ascending sequence order and keep the FIRST record in input order if a sequence repeats. Ignore sequences <= checkpoint. A delete of a missing key is a no-op. Return {'state': recovered dictionary, 'last_sequence': greatest applied sequence or checkpoint if none}. Gaps are allowed and are not errors; do not mutate the snapshot. Examples (JSON; null means Python None): Input: {"snapshot": {"a": 1}, "checkpoint": 2, "wal": [[4, "set", "a", 3], [3, "set", "b", 2]]} Output: {"state": {"a": 3, "b": 2}, "last_sequence": 4} Input: {"snapshot": {"a": 7}, "checkpoint": 3, "wal": [[2, "set", "a", 0], [4, "delete", "a", null], [4, "set", "a", 9]]} Output: {"state": {}, "last_sequence": 4}

def replay_checkpoint(snapshot: dict[str, int], checkpoint: int, wal: list[list]) -> dict:
    """Recover a snapshot by replaying later unique log records."""
    pass