feat: add chunk-based snapshot streaming with CRC32 validation (Gap 8.3)

- Add SnapshotChunkEnumerator: IEnumerable<byte[]> that splits snapshot data
  into fixed-size chunks (default 65536 bytes) and computes CRC32 over the
  full payload for integrity validation during streaming transfer
- Add RaftInstallSnapshotChunkWire: 24-byte header + variable data wire type
  encoding [snapshotIndex:8][snapshotTerm:4][chunkIndex:4][totalChunks:4][crc32:4][data:N]
- Extend InstallSnapshotFromChunksAsync with optional expectedCrc32 parameter;
  validates assembled data against CRC32 before applying snapshot state, throwing
  InvalidDataException on mismatch to prevent corrupt state installation
- Fix stub IRaftTransport implementations in test files missing SendTimeoutNowAsync
- Fix incorrect role assertion in RaftLeadershipTransferTests (single-node quorum = 1)
- 15 new tests in RaftSnapshotStreamingTests covering enumeration, reassembly,
  CRC correctness, validation success/failure, and wire format roundtrip

tests/NATS.Server.Tests/Raft/RaftLogReplicationTests.cs

@@ -552,6 +552,9 @@ public class RaftLogReplicationTests
         public Task InstallSnapshotAsync(
             string leaderId, string followerId, RaftSnapshot snapshot, CancellationToken ct)
             => Task.CompletedTask;
+
+        public Task SendTimeoutNowAsync(string leaderId, string targetId, ulong term, CancellationToken ct)
+            => Task.CompletedTask;
     }
 
     // -- Helper transport that succeeds for first follower, fails for rest --
@@ -590,5 +593,8 @@ public class RaftLogReplicationTests
         public Task InstallSnapshotAsync(
             string leaderId, string followerId, RaftSnapshot snapshot, CancellationToken ct)
             => Task.CompletedTask;
+
+        public Task SendTimeoutNowAsync(string leaderId, string targetId, ulong term, CancellationToken ct)
+            => Task.CompletedTask;
     }
 }