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/src/MoShiro.Backend.Application/Updates/BackendUpdates.cs

@@ -21,7 +21,7 @@ public interface IDatabaseBackup
 public sealed record BackendUpdateView(string Channel, bool Enabled, string State, string? CurrentTag, string? CurrentDigest, ReleaseOffer? Available, string? Error);
 
 /// <summary>
-/// Backend 的发布通知和确认，和 Agent、Probe 是同一种更新：面板登记 CI 签名的清单，管理员确认之后 Backend 有序退出，
+/// Backend 的发布通知和确认，和 Agent 是同一种更新：面板登记 CI 签名的清单，管理员确认之后 Backend 有序退出，
 /// Quadlet 单元每次启动都先拉最新镜像（<c>Pull=always</c>），所以退出再被拉起就是更新。Backend 自己不下载、不替换任何东西。
 /// 只有设了环境变量 <c>MOSHIRO_UPDATE=restart</c>（Quadlet 模板里写好了）才启用；别的部署方式退出之后不会重新拉取。
 /// 只提示不自动重启：Backend 指挥所有节点，重启会让它们全部断一两分钟。