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/Net/QuicAlpn.cs

@@ -1,6 +1,6 @@
 namespace MoShiro.Measurement;
 
-/// <summary>The application protocol negotiated by the Agent and Probe QUIC endpoints.</summary>
+/// <summary>The application protocol negotiated by the Agent and 拨号方 QUIC endpoints.</summary>
 internal static class QuicAlpn
 {
     internal static readonly byte[] Value = "h3"u8.ToArray();