feat(quic): replace probe UDP data plane with QUIC datagrams

backend/shared/MoShiro.Measurement/Core/Contracts.cs

@@ -11,6 +11,8 @@ namespace MoShiro.Measurement;
 /// </summary>
 public class DataSession
 {
+    private IDatagramConnection? _datagramConnection;
+
     public DataSession(long sessionId, byte[] key)
     {
         SessionId = sessionId;
@@ -21,13 +23,26 @@ public class DataSession
 
     public byte[] Key { get; }
 
+    public IDatagramConnection? DatagramConnection => Volatile.Read(ref _datagramConnection);
+
+    public void BindDatagramConnection(IDatagramConnection connection) => Volatile.Write(ref _datagramConnection, connection);
+
     /// <summary>对端最近一次通过认证的 UDP 源地址（监听方把单向数据发往这里；拨号方不需要）。</summary>
-    public virtual IPEndPoint? UdpEndpoint => null;
+    public virtual IPEndPoint? DatagramRemoteEndPoint => null;
 
     /// <summary>TCP 首帧的随机数只能用一次。拨号方不接受连接，所以默认放行。</summary>
     public virtual bool UseNonce(ReadOnlySpan<byte> nonce) => true;
 }
 
+/// <summary>Unreliable QUIC datagram sender bound to one authenticated measurement session.</summary>
+public interface IDatagramConnection
+{
+    bool Send(ReadOnlySpan<byte> datagram);
+}
+
+/// <summary>Transport-layer QUIC counters. These are separate from application Datagram delivery/loss.</summary>
+public sealed record QuicTransportSnapshot(double? PacketLossRate, ulong PacketsSent, uint CongestionEvents, uint EcnCongestionEvents, ushort PathMtu);
+
 public interface IDataSessions
 {
     DataSession? Find(long sessionId);
@@ -60,7 +75,7 @@ public enum Admission
 public interface IAdmission
 {
     /// <summary>UDP 包：<paramref name="source"/> 是认证通过的源地址（实现可以据此学习 NAT 重绑定）。</summary>
-    Admission AdmitPacket(DataSession session, in UdpHeader header, SocketAddress source, long nowUs);
+    Admission AdmitDatagram(DataSession session, in MeasurementDatagramHeader header, SocketAddress source, long nowUs);
 
     /// <summary>TCP 回显的一条消息（只限速）。</summary>
     bool AdmitMessage(DataSession session, uint streamId, long nowUs);
@@ -201,7 +216,7 @@ public sealed class BlockEnv
     /// <summary>当前的数据面会话（拨号方固定；监听方是 Agent 当前的 Probe 会话，可能没有）。</summary>
     public required Func<DataSession?> Session { get; init; }
 
-    public UdpMux? Udp { get; init; }
+    public QuicDatagramTransport? Datagrams { get; init; }
 
     /// <summary>监听方的准入策略（限速等）；拨号方为 null。</summary>
     public IAdmission? Admission { get; init; }
@@ -238,7 +253,7 @@ public sealed class BlockEnv
             Clock = Clock,
             Log = Log,
             Session = () => session,
-            Udp = Udp,
+            Datagrams = Datagrams,
             Admission = Admission,
             Counters = Counters,
             TcpListen = TcpListen,