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/Core/RunningBlock.cs

@@ -1,6 +1,6 @@
 namespace MoShiro.Measurement;
 
-/// <summary>一个运行中的积木：自己的取消令牌和后台任务。Probe 和 Agent 共用。</summary>
+/// <summary>一个运行中的积木：自己的取消令牌和后台任务。拨号方和监听方共用。</summary>
 public sealed class RunningBlock : IDisposable
 {
     private readonly CancellationTokenSource _cts = new();