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/Platform/HostSampler.cs

@@ -4,7 +4,7 @@ using MoShiro.Protocol;
 namespace MoShiro.Measurement;
 
 /// <summary>
-/// Agent 和 Probe 共用。主机资源占用的只读快照：CPU 占用、内存、根分区、网卡累计流量与瞬时网速、系统运行时长。
+/// 拨号方和监听方共用。主机资源占用的只读快照：CPU 占用、内存、根分区、网卡累计流量与瞬时网速、系统运行时长。
 /// 只有汇总的数字：<b>不采集 CPU 型号、主机名、网卡名和任何 IP 地址</b>，所以面板和展示页想泄漏也泄漏不出去。
 /// Linux 读 <c>/proc</c> 和 <c>/sys</c>；容器通过 HOST_PROC、HOST_SYS、HOST_ROOT 指定宿主路径。
 /// Windows 用 GetSystemTimes、GlobalMemoryStatusEx 和 NetworkInterface（见 WindowsHost）；macOS 等其他系统只有磁盘和运行时长。读不到的项是 null。不执行任何命令，不写文件。