17731e2af5
Add MessageRecord class with Encode/Decode for the binary wire format used in JetStream file store blocks. Uses varint-encoded lengths, XxHash64 checksums, and a flags byte for deletion markers. Go reference: filestore.go:6720-6724, 8180-8250, 8770-8777 13 tests covering round-trip, headers, checksum validation, corruption detection, varint encoding, deleted flag, empty/large payloads.