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.
Replace Deflate+XOR with IronSnappy S2 block compression and ChaCha20-Poly1305 / AES-256-GCM
AEAD encryption, matching golang/nats-server/server/filestore.go. Introduces FSV2 envelope
format alongside existing FSV1 for backward compatibility. Adds 55 new tests across
S2CodecTests, AeadEncryptorTests, and FileStoreV2Tests covering all 6 cipher×compression
permutations, tamper detection, and legacy format round-trips.
Adds Microsoft.Extensions.Hosting.WindowsServices package and a --service
CLI flag to Program.cs that logs service mode activation, enabling future
Windows Service lifecycle management.
