[events] Cold segment reader #657

Implement reading and querying a single cold segment from disk. Given a segment directory, support:

• Open: load MPHF from index.hash, load ledger offset array from events.pack app data
• Index lookup: term key → MPHF slot → fingerprint check → bitmap deserialize from index.pack
• Event fetch: event ID → record index → decompress record from events.pack → extract event
• Full query: lookup bitmaps → intersect → trim to ledger range → fetch events → post-filter

Add tests that write a segment (using cold segment writer), then query it with various filter combinations.