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/ControlBackoff.cs

@@ -0,0 +1,31 @@
+namespace MoShiro.Agent;
+
+/// <summary>Reconnect cadence for the control connection of a single dialer-to-listener path: 1, 2, 4, 8, 16, then five 32-second waits, then once per minute.</summary>
+public sealed class ControlBackoff
+{
+    private static readonly TimeSpan[] s_delays =
+    [
+        TimeSpan.FromSeconds(1),
+        TimeSpan.FromSeconds(2),
+        TimeSpan.FromSeconds(4),
+        TimeSpan.FromSeconds(8),
+        TimeSpan.FromSeconds(16),
+        TimeSpan.FromSeconds(32),
+        TimeSpan.FromSeconds(32),
+        TimeSpan.FromSeconds(32),
+        TimeSpan.FromSeconds(32),
+        TimeSpan.FromSeconds(32),
+    ];
+
+    private int _attempt;
+
+    public int Attempts => Volatile.Read(ref _attempt);
+
+    public TimeSpan Next()
+    {
+        int attempt = Interlocked.Increment(ref _attempt) - 1;
+        return attempt < s_delays.Length ? s_delays[attempt] : TimeSpan.FromMinutes(1);
+    }
+
+    public void Reset() => Interlocked.Exchange(ref _attempt, 0);
+}