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/agent/src/MoShiro.Agent/Dial/QuicRedialBackoff.cs

@@ -0,0 +1,23 @@
+namespace MoShiro.Agent;
+
+/// <summary>
+/// QUIC 数据通道的重拨节奏：1、2、3、5 秒，之后固定 5 秒。数据通道断开期间的测量是缺口，所以不像控制连接那样拉长到一分钟。
+/// 抖动由调用方通过 <paramref name="random"/> 给出，避免多条线路同时重拨。
+/// </summary>
+public sealed class QuicRedialBackoff
+{
+    private static readonly TimeSpan[] s_delays = [TimeSpan.FromSeconds(1), TimeSpan.FromSeconds(2), TimeSpan.FromSeconds(3), TimeSpan.FromSeconds(5)];
+
+    private int _attempt;
+
+    public int Attempts => _attempt;
+
+    public TimeSpan Next(Random? random = null)
+    {
+        TimeSpan delay = s_delays[Math.Min(_attempt, s_delays.Length - 1)];
+        _attempt++;
+        return random is null ? delay : delay * (0.8 + (0.4 * random.NextDouble()));
+    }
+
+    public void Reset() => _attempt = 0;
+}