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/tests/MoShiro.Backend.Analysis.Tests/ModelAndTestEvaluatorTests.cs

@@ -111,7 +111,7 @@ public sealed class ModelAndTestEvaluatorTests
     [Fact]
     public void TestEvaluator_Hy2Ladder_APhaseSplitIntoSeveralChunksIsOneStep()
     {
-        // 真实的 Probe 把一个阶段切成几块（一块最多 50000 条记录）：阶梯里的一步是一个阶段，不是一个块
+        // 真实的拨号侧 Agent 把一个阶段切成几块（一块最多 50000 条记录）：阶梯里的一步是一个阶段，不是一个块
         RawChunk[] chunks =
         [
             ChunkFactory.Train(s_agent, T0, 10, 0),