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.Protocol/Wire/TcpWire.cs

@@ -6,7 +6,7 @@ using Fleet.Core;
 namespace MoShiro.Protocol;
 
 /// <summary>
-/// 数据面的 TCP 连接（Fleet 之外的线格式）。连接都由 Probe 发起；TLS 是可选的（ClientHello 以 0x16 开头，
+/// 数据面的 TCP 连接（Fleet 之外的线格式）。连接都由 拨号方 发起；TLS 是可选的（ClientHello 以 0x16 开头，
 /// 明文首帧以 "MN" 开头，所以同一个端口可以区分）。首帧：
 /// <c>"MN"(2) version(1) session_id(8) stream_id(4) nonce(16) mac(16)</c>，认证失败直接关闭连接；
 /// 之后是 <c>VarInt 长度 | 正文</c> 的消息，回显类消息的正文是 <c>seq(u32) send_ts_us(i64) | padding</c>。