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/BackendHost.cs

@@ -38,7 +38,7 @@ public static class BackendHost
         IServiceCollection services = builder.Services;
         services.AddSingleton(time ?? TimeProvider.System);
         services.AddMoShiroSqlite(options.DataDirectory);
-        // 国家和运营商：Backend 用节点的来源地址查一次（Probe、Agent 不访问任何外部服务）；地址留空就不查
+        // 国家和运营商：Backend 用节点的来源地址查一次（拨号方、Agent 不访问任何外部服务）；地址留空就不查
         if (string.IsNullOrWhiteSpace(options.GeoLookupUrl))
         {
             services.AddSingleton<IGeoLookup, NoGeoLookup>();

backend/src/MoShiro.Backend/Program.cs

@@ -1,7 +1,7 @@
 using MoShiro.Backend;
 
 WebApplication app = BackendHost.Build(args);
-// 启动时打一行构建信息：版本、tag、提交、构建时间（和 Agent、Probe 启动时打的是同一组）
+// 启动时打一行构建信息：版本、tag、提交、构建时间（和 Agent 启动时打的是同一组）
 if (app.Logger.IsEnabled(LogLevel.Information))
 {
     app.Logger.LogInformation("moshiro backend starting: version {BuildInfo}", MoShiro.Protocol.BuildInfo.Summary);