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/Tcp.cs

@@ -223,7 +223,7 @@ public sealed class TcpAcceptor
                 using var authCts = CancellationTokenSource.CreateLinkedTokenSource(ct);
                 authCts.CancelAfter(s_authTimeout);
 
-                // 预握手敲门：名册里的 Probe 才算得出敲门包；不通过就不写任何字节、直接断开
+                // 预握手敲门：名册里的 拨号方 才算得出敲门包；不通过就不写任何字节、直接断开
                 if (!await _knock.AdmitAsync(stream, (client.Client.RemoteEndPoint as IPEndPoint)?.Address, authCts.Token))
                 {
                     _counters.DropInvalid();