agent: unify listening and dialing with directed mutual measurements

Replace Probe with Agent across node runtime, Fleet contracts, backend state, API and storage. Report incoming sessions and outgoing link health through each Agent uplink, and retain directed link controls and retry.

Update protocol fixtures and end-to-end coverage for the unified model; accept requested cancellation during the Fleet timeout test cleanup.

backend/shared/MoShiro.Measurement/Net/TcpInfo.cs

@@ -71,7 +71,7 @@ public static class TcpInfo
 
 /// <summary>
 /// 一条连接上周期性的内核采样（<c>tcp_info</c> 流）：每个采样一行 (ts, side, srtt, rttvar, cwnd, retrans_total, segs_out, bytes_acked, conn)。
-/// 两端各采样自己的套接字：Probe 的重传反映上行方向，Agent 的重传反映下行方向（代理场景里大流量走这个方向）。
+/// 两端各采样自己的套接字：拨号方 的重传反映上行方向，Agent 的重传反映下行方向（代理场景里大流量走这个方向）。
 /// 累计计数器在 Backend 按 (side, conn) 分别求差；conn 是当前记录器内分配的连接编号，不包含网络地址。
 /// </summary>
 public sealed class TcpInfoRecorder
@@ -84,7 +84,7 @@ public sealed class TcpInfoRecorder
     {
         Rows = new RowStore(spec, side, 4096);
         _clock = clock;
-        _side = side == Side.Probe ? 0 : 1;
+        _side = side == Side.Dialer ? 0 : 1;
     }
 
     public RowStore Rows { get; }