[events] Freeze (hot → cold) #661

Implement the segment freeze process:

1. Start new hot segment immediately (new segment accepts writes)
2. Build cold segment from old hot segment using cold segment writer
   • If using LiveWriter: Freeze() in-place, then build index files
   • If using flat files/RocksDB: read events, produce cold segment
3. Old hot segment continues serving reads during freeze
4. Mark segment as frozen in DB atomically after all cold files are written
5. Discard old hot segment files and in-memory index

Add tests for: freeze produces valid cold segment, concurrent reads during freeze, two hot segments coexist briefly.