feat: add S2 compression and AEAD encryption for FileStore (Go parity)

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.

tests/NATS.Server.Tests/JetStream/Storage/S2CodecTests.cs

@@ -0,0 +1,154 @@
+// Reference: golang/nats-server/server/filestore.go
+// Go uses S2/Snappy compression throughout FileStore:
+//   - msgCompress / msgDecompress (filestore.go ~line 840)
+//   - compressBlock / decompressBlock for block-level data
+// These tests verify the .NET S2Codec helper used in the FSV2 envelope path.
+
+using NATS.Server.JetStream.Storage;
+using System.Text;
+
+namespace NATS.Server.Tests.JetStream.Storage;
+
+public sealed class S2CodecTests
+{
+    // Go: TestFileStoreBasics (S2 permutation) filestore_test.go:86
+    [Fact]
+    public void Compress_then_decompress_round_trips()
+    {
+        var original = "Hello, NATS JetStream S2 compression!"u8.ToArray();
+
+        var compressed = S2Codec.Compress(original);
+        var restored = S2Codec.Decompress(compressed);
+
+        restored.ShouldBe(original);
+    }
+
+    [Fact]
+    public void Compress_empty_returns_empty()
+    {
+        var compressed = S2Codec.Compress([]);
+        compressed.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public void Decompress_empty_returns_empty()
+    {
+        var decompressed = S2Codec.Decompress([]);
+        decompressed.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public void Compress_large_highly_compressible_payload()
+    {
+        // 1 MB of repeated 'A' — highly compressible.
+        var original = new byte[1024 * 1024];
+        Array.Fill(original, (byte)'A');
+
+        var compressed = S2Codec.Compress(original);
+        var restored = S2Codec.Decompress(compressed);
+
+        // S2/Snappy should compress this well.
+        compressed.Length.ShouldBeLessThan(original.Length);
+        restored.ShouldBe(original);
+    }
+
+    [Fact]
+    public void Compress_large_incompressible_payload_round_trips()
+    {
+        // 1 MB of random data — not compressible, but must still round-trip.
+        var original = new byte[1024 * 1024];
+        Random.Shared.NextBytes(original);
+
+        var compressed = S2Codec.Compress(original);
+        var restored = S2Codec.Decompress(compressed);
+
+        restored.ShouldBe(original);
+    }
+
+    [Fact]
+    public void Compress_single_byte_round_trips()
+    {
+        var original = new byte[] { 0x42 };
+        var compressed = S2Codec.Compress(original);
+        var restored = S2Codec.Decompress(compressed);
+        restored.ShouldBe(original);
+    }
+
+    [Fact]
+    public void Compress_binary_all_byte_values_round_trips()
+    {
+        var original = new byte[256];
+        for (var i = 0; i < 256; i++)
+            original[i] = (byte)i;
+
+        var compressed = S2Codec.Compress(original);
+        var restored = S2Codec.Decompress(compressed);
+        restored.ShouldBe(original);
+    }
+
+    // Go: msgCompress with trailing CRC (filestore.go ~line 840) — the checksum
+    // lives outside the S2 frame so only the body is compressed.
+    [Fact]
+    public void CompressWithTrailingChecksum_preserves_last_n_bytes_uncompressed()
+    {
+        const int checksumSize = 8;
+        var body = Encoding.UTF8.GetBytes("NATS payload body that should be compressed");
+        var checksum = new byte[checksumSize];
+        Random.Shared.NextBytes(checksum);
+
+        var input = body.Concat(checksum).ToArray();
+
+        var result = S2Codec.CompressWithTrailingChecksum(input, checksumSize);
+
+        // Last checksumSize bytes must be verbatim.
+        var resultChecksum = result[^checksumSize..];
+        resultChecksum.ShouldBe(checksum);
+    }
+
+    [Fact]
+    public void CompressWithTrailingChecksum_zero_checksum_compresses_all()
+    {
+        var data = "Hello, no checksum"u8.ToArray();
+        var result = S2Codec.CompressWithTrailingChecksum(data, 0);
+        var restored = S2Codec.Decompress(result);
+        restored.ShouldBe(data);
+    }
+
+    [Fact]
+    public void DecompressWithTrailingChecksum_round_trips()
+    {
+        const int checksumSize = 8;
+        var body = new byte[512];
+        Random.Shared.NextBytes(body);
+        var checksum = new byte[checksumSize];
+        Random.Shared.NextBytes(checksum);
+
+        var input = body.Concat(checksum).ToArray();
+
+        var compressed = S2Codec.CompressWithTrailingChecksum(input, checksumSize);
+        var restored = S2Codec.DecompressWithTrailingChecksum(compressed, checksumSize);
+
+        restored.ShouldBe(input);
+    }
+
+    [Fact]
+    public void CompressWithTrailingChecksum_empty_input_returns_empty()
+    {
+        var result = S2Codec.CompressWithTrailingChecksum([], 0);
+        result.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public void CompressWithTrailingChecksum_negative_size_throws()
+    {
+        Should.Throw<ArgumentOutOfRangeException>(
+            () => S2Codec.CompressWithTrailingChecksum([1, 2, 3], -1));
+    }
+
+    [Fact]
+    public void DecompressWithTrailingChecksum_negative_size_throws()
+    {
+        Should.Throw<ArgumentOutOfRangeException>(
+            () => S2Codec.DecompressWithTrailingChecksum([1, 2, 3], -1));
+    }
+}