fix: prevent QUIC heartbeat stalls and isolate TCP retransmission counters [CI] [SKIP TESTS]

backend/shared/MoShiro.Protocol/Wire/TcpWire.cs

@@ -1,3 +1,4 @@
+using System.Buffers;
 using System.Buffers.Binary;
 using System.Security.Cryptography;
 using Fleet.Core;
@@ -124,9 +125,18 @@ public static class TcpWire
 
     public static async ValueTask WriteMessageAsync(Stream stream, ReadOnlyMemory<byte> body, CancellationToken ct)
     {
-        byte[] prefix = new byte[VarInt.MaxBytes];
-        int n = VarInt.Write(prefix, (ulong)body.Length);
-        await stream.WriteAsync(prefix.AsMemory(0, n), ct);
-        await stream.WriteAsync(body, ct);
+        byte[] frame = ArrayPool<byte>.Shared.Rent(checked(VarInt.MaxBytes + body.Length));
+        try
+        {
+            int n = VarInt.Write(frame, (ulong)body.Length);
+            body.CopyTo(frame.AsMemory(n));
+            // TCP_NODELAY is enabled: two writes otherwise create a tiny
+            // length-only segment before every echo body (and two TLS records).
+            await stream.WriteAsync(frame.AsMemory(0, n + body.Length), ct);
+        }
+        finally
+        {
+            ArrayPool<byte>.Shared.Return(frame);
+        }
     }
 }