[events] In-memory bitmap index

[urvisavla]

Implement the in-memory index using roaring bitmaps: a concurrent map of 16-byte term key → roaring bitmap pointer, protected by a read-write lock. Includes:

• Term key computation: hash(value bytes || field byte) → 16 bytes
• Add(termKey, eventID) for ingestion
• Lookup(termKey) → bitmap for queries

Out of scope:

• AND/OR intersection across multiple bitmaps
• Ledger range trimming using the ledger offset array
• Ascending and descending iteration over matching event IDs