Stage 4: Compaction

Flush enough and you drown in segments: every read scans more files, and deleted data still occupies disk under tombstones. Compaction is the background job that merges segments — keeping only each key's newest value and dropping tombstones once nothing older can resurrect the key. It's the "tree" in LSM-tree, and tuning it is half of operating RocksDB or Cassandra.

Implement store.compact():

• merge all segments into a single sorted segment (newest value per key wins)
• drop tombstones entirely — after a full compaction nothing older exists, so the marker is no longer needed
• store.segments ends as [mergedSegment] (or [] if everything was deleted)
• reads must behave identically before and after