LSM-tree compaction

Implement LSM-tree compaction. You are given 'runs', a list of sorted runs ordered oldest-first to newest-last; each run is a list of [key, seq, value] entries sorted ascending by key, where value is a string or the literal string 'TOMBSTONE' for a delete. Higher seq means newer. Merge all runs into one sorted output keeping, for each key, only the entry with the largest seq; if that winning entry is a tombstone, the key is dropped entirely (deleted). Return the compacted run as a list of [key, seq, value] sorted ascending by key, excluding deleted keys.