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/MonoClock.cs

@@ -1,6 +1,6 @@
 namespace MoShiro.Measurement;
 
-/// <summary>节点的单调时钟（微秒）：t1…t4、t_send、t_recv 都用它；Probe 和 Agent 各自一个，块头里的时钟锚点把它对应到 UTC。</summary>
+/// <summary>节点的单调时钟（微秒）：t1…t4、t_send、t_recv 都用它；每个节点各一个，块头里的时钟锚点把它对应到 UTC。</summary>
 public sealed class MonoClock
 {
     private readonly TimeProvider _time;