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/Blocks/Session.cs

@@ -19,7 +19,7 @@ public sealed class IdleListener : BlockBase
 
 /// <summary>
 /// 反复建立连接（TCP，可选 TLS）并记录每个阶段的耗时和结果。结果编码与 Backend 的 <c>ConnectOutcome</c> 一致：
-/// 0 成功、1 超时、2 被重置或拒绝、3 TLS 告警、4 其他。拨号方是 Probe 时对端是 Agent（按指纹固定证书，建好连接即关闭）；
+/// 0 成功、1 超时、2 被重置或拒绝、3 TLS 告警、4 其他。拨号方连接监听方时（按指纹固定证书，建好连接即关闭）；
 /// 作为目标任务在 Agent 上运行时对端是任意地址（不校验证书，只测握手耗时）。
 /// </summary>
 public sealed class ConnectDialer : BlockBase
@@ -34,7 +34,7 @@ public sealed class ConnectDialer : BlockBase
     public ConnectDialer(BlockEnv env, BlockSpec spec)
         : base(env, spec)
     {
-        _log = new DriverLog(spec.Kind.FindStream("conn")!, Side.Probe, 1024);
+        _log = new DriverLog(spec.Kind.FindStream("conn")!, Side.Dialer, 1024);
         _attempt = _log.Store.ColumnIndex("attempt");
         _ts = _log.Store.ColumnIndex("ts");
         _tcp = _log.Store.ColumnIndex("tcp_us");
@@ -147,7 +147,7 @@ public sealed class HoldDialer : BlockBase
     public HoldDialer(BlockEnv env, BlockSpec spec)
         : base(env, spec)
     {
-        _log = new DriverLog(spec.Kind.FindStream("hold")!, Side.Probe, 1024);
+        _log = new DriverLog(spec.Kind.FindStream("hold")!, Side.Dialer, 1024);
         _ts = _log.Store.ColumnIndex("ts");
         _event = _log.Store.ColumnIndex("event");
         _lifetime = _log.Store.ColumnIndex("lifetime_ms");
@@ -293,7 +293,7 @@ public sealed class IcmpEchoSolo : BlockBase
     public IcmpEchoSolo(BlockEnv env, BlockSpec spec)
         : base(env, spec)
     {
-        _log = new DriverLog(spec.Kind.FindStream("msg")!, Side.Probe, 1024);
+        _log = new DriverLog(spec.Kind.FindStream("msg")!, Side.Dialer, 1024);
         _seq = _log.Store.ColumnIndex("seq");
         _size = _log.Store.ColumnIndex("size");
         _t1 = _log.Store.ColumnIndex("t1");