Implements announce-based KeyPackage distribution for serverless MLS:
- MeshAnnounce now includes optional `keypackage_hash` field (8 bytes)
- CAP_MLS_READY capability flag for nodes with KeyPackages
- KeyPackageCache for storing received KeyPackages:
- Indexed by mesh address
- Multiple per address (for rotation)
- TTL-based expiry
- Capacity-bounded with LRU eviction
- Mesh protocol messages:
- KeyPackageRequest (request by address or hash)
- KeyPackageResponse (KeyPackage + hash)
- KeyPackageUnavailable (negative response)
Protocol flow:
1. Bob announces with keypackage_hash
2. Alice requests KeyPackage via mesh
3. Bob (or relay) responds with full KeyPackage
4. Alice creates MLS Welcome, sends to Bob via mesh
