[events] Hot segment writer

[urvisavla]

Implement the writer for the hot segment — the active-ingest store that owns one Chunk's events while it's being filled.

Scope:

• Per-ledger ingest: write events to storage, update in-memory bitmap mirror, update ledger offset array — all atomic in one batch.
• Up-front validation: ledger must be in the chunk's range AND must be the next-expected ledger (no duplicates, no gaps).
• Per-ledger commit with fsync and durability guarantees.
• Open / Close lifecycle; warmup on reopen to reconstruct the in-memory mirrors from the on-disk caches.

Out of scope (separate issues):

• Wiring the writer into the live ingestion pipeline ([events] Wire into ingestion pipeline #664).
• Freeze (hot → cold transition) ([events] Freeze (hot → cold) #661).

Add tests for write correctness, per-ledger durability, sequence validation, and reopen-after-close recovery.