[events] Hot segment reader #660

Implement the read primitives for the hot segment, against the storage backend used by the writer (#659).

Scope:

• Lookup(termKey) → bitmap of event IDs (returns ErrTermNotFound on miss).
• FetchEvents([]eventID) → iter of decoded payloads.
• All() → iter over every event in the chunk in event-id order.
• Metadata accessors: ChunkID, EventCount, Offsets.
• Concurrency: many concurrent readers; writer holds an exclusive mutex during ingest.

Out of scope (separate issues):

• Bitmap intersection, ledger-range trimming, and post-filter across multiple segments — that's the segment-coordination layer ([events] Single-chunk query engine #663).
• Wiring into the query handler (part of [events] Single-chunk query engine #663).

Add tests covering: lookup hits and misses, ordered streaming of all events, fetch-by-id round-trips, and concurrent-read correctness.