web: show how long a line has been out of contact

The Backend now keeps the time since a link's control layer was last up, because retries move the control state in and out of backoff and its own timestamp keeps resetting. The overview exposes it as control_down_since and the card shows it next to the lost-contact notice.

backend/tests/MoShiro.Backend.Tests/LinkRuntimeTests.cs

@@ -0,0 +1,28 @@
+using MoShiro.Backend.Application;
+using MoShiro.Protocol;
+
+namespace MoShiro.Backend.Tests;
+
+public sealed class LinkRuntimeTests
+{
+    private static LinkHealth Health(ControlState state) => new() { ListenerId = "agt_l", Control = { State = state } };
+
+    [Fact]
+    public void DownSince_StaysAtTheFirstLostReport_AndClearsWhenTheControlLinkIsUp()
+    {
+        var runtime = new LinkRuntime();
+        runtime.Replace("agt_d", 1_000, [Health(ControlState.Up)]);
+        Assert.Null(runtime.Get("agt_d", "agt_l", IpVersions.V4)!.DownSinceMs);
+
+        runtime.Replace("agt_d", 2_000, [Health(ControlState.Backoff)]);
+        runtime.Replace("agt_d", 3_000, [Health(ControlState.Dialing)]); // 重试期间状态在退避和拨号之间来回，失联的起点不变
+        runtime.Replace("agt_d", 4_000, [Health(ControlState.Backoff)]);
+        Assert.Equal(2_000, runtime.Get("agt_d", "agt_l", IpVersions.V4)!.DownSinceMs);
+
+        runtime.MarkDialerOffline("agt_d", 5_000);
+        Assert.Equal(2_000, runtime.Get("agt_d", "agt_l", IpVersions.V4)!.DownSinceMs);
+
+        runtime.Replace("agt_d", 6_000, [Health(ControlState.Up)]);
+        Assert.Null(runtime.Get("agt_d", "agt_l", IpVersions.V4)!.DownSinceMs);
+    }
+}