[events] Cold segment writer #656

Build the component that produces a complete cold segment directory from events and bitmaps. Inputs: encoded events, in-memory bitmaps, ledger offset array. Outputs: a segment directory containing:

• events.pack — compressed events via packfile Writer, with ledger offset array embedded as app data
• index.hash — MPHF built from all term keys
• index.pack — serialized bitmaps, one per term, each prefixed with 4-byte fingerprint

Used by both freeze and backfill. Add tests that produce a segment directory and verify file contents.