feat(cluster): add MetaSnapshotCodec with S2 compression and versioned format

Implements binary codec for meta-group snapshots: 2-byte little-endian version
header followed by S2-compressed JSON of the stream assignment map. Adds
[JsonObjectCreationHandling(Populate)] to StreamAssignment.Consumers so the
getter-only dictionary is populated in-place during deserialization. 8 tests
covering round-trip, compression ratio, field fidelity, multi-consumer restore,
version rejection, and truncation guard.

Go reference: jetstream_cluster.go:2075-2145 (encodeMetaSnapshot/decodeMetaSnapshot)

src/NATS.Server/JetStream/Cluster/MetaSnapshotCodec.cs

@@ -0,0 +1,60 @@
+using System.Buffers.Binary;
+using System.Text.Json;
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream.Cluster;
+
+/// <summary>
+/// Binary codec for meta-group snapshots.
+/// Format: [2:version_le][N:S2-compressed JSON of assignment map]
+/// Go reference: jetstream_cluster.go:2075-2145 (encodeMetaSnapshot/decodeMetaSnapshot)
+/// </summary>
+internal static class MetaSnapshotCodec
+{
+    private const ushort CurrentVersion = 1;
+
+    // Use Populate so the getter-only Consumers dictionary on StreamAssignment
+    // is populated in-place by the deserializer rather than requiring a setter.
+    // Go reference: jetstream_cluster.go streamAssignment consumers map restoration.
+    private static readonly JsonSerializerOptions SerializerOptions = new()
+    {
+        PreferredObjectCreationHandling = System.Text.Json.Serialization.JsonObjectCreationHandling.Populate,
+    };
+
+    /// <summary>
+    /// Encodes <paramref name="assignments"/> into the versioned, S2-compressed binary format.
+    /// Go reference: jetstream_cluster.go:2075 encodeMetaSnapshot.
+    /// </summary>
+    public static byte[] Encode(Dictionary<string, StreamAssignment> assignments)
+    {
+        var json = JsonSerializer.SerializeToUtf8Bytes(assignments, SerializerOptions);
+        var compressed = S2Codec.Compress(json);
+
+        var result = new byte[2 + compressed.Length];
+        BinaryPrimitives.WriteUInt16LittleEndian(result, CurrentVersion);
+        compressed.CopyTo(result, 2);
+        return result;
+    }
+
+    /// <summary>
+    /// Decodes a versioned, S2-compressed binary snapshot into a stream assignment map.
+    /// Go reference: jetstream_cluster.go:2100 decodeMetaSnapshot.
+    /// </summary>
+    /// <exception cref="InvalidOperationException">
+    /// Thrown when <paramref name="data"/> is too short or contains an unrecognised version.
+    /// </exception>
+    public static Dictionary<string, StreamAssignment> Decode(byte[] data)
+    {
+        if (data.Length < 2)
+            throw new InvalidOperationException("Meta snapshot too short to contain version header.");
+
+        var version = BinaryPrimitives.ReadUInt16LittleEndian(data);
+        if (version != CurrentVersion)
+            throw new InvalidOperationException($"Unknown meta snapshot version: {version}");
+
+        var compressed = data.AsSpan(2);
+        var json = S2Codec.Decompress(compressed);
+        return JsonSerializer.Deserialize<Dictionary<string, StreamAssignment>>(json, SerializerOptions)
+            ?? new Dictionary<string, StreamAssignment>();
+    }
+}