Write-ahead log compaction

A write-ahead log records mutations to a key-value store. You are given an ordered list of log entries, each either ["put", key, value] or ["del", key]. Compaction replays the log to compute the final materialized state, where each key's value is whatever the last put for it set (a later del removes the key entirely; a put after a del re-adds it). Return the compacted state as a list of [key, value] pairs sorted by key ascending. Keys are strings; values are integers.