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/openapi/v1.json

@@ -8774,7 +8774,8 @@
           "control_state": {
             "type": "string",
             "default": "dialing"
-          }
+          },
+          "control_down_since": {}
         }
       },
       "RawChunkMetaDto": {

backend/src/MoShiro.Backend.Api/Common/Mappers.cs

@@ -53,7 +53,7 @@ public static class Mappers
                 QuicState.Retrying => "retry_wait",
                 QuicState.Idle => "idle",
                 _ => "connecting",
-            }, quic.RetryAttempt, quic.RetryInMs, ControlStateName(link.Control.State));
+            }, quic.RetryAttempt, quic.RetryInMs, ControlStateName(link.Control.State), Time(snapshot.DownSinceMs));
     }
 
     private static string ControlStateName(ControlState state) => state switch

backend/src/MoShiro.Backend.Api/Contracts/PublicDtos.cs

@@ -52,7 +52,8 @@ public sealed record QuicLineHealthDto(
     string State = "connecting",
     int RetryAttempt = 0,
     long? RetryInMs = null,
-    string ControlState = "dialing");
+    string ControlState = "dialing",
+    DateTimeOffset? ControlDownSince = null);
 
 public sealed record OverviewDto(string Range, DateTimeOffset From, DateTimeOffset To, string Step, string Source, bool Degraded, IReadOnlyList<OverviewLineDto> Lines);
 

backend/src/MoShiro.Backend.Application/Agents/LinkRuntime.cs

@@ -3,7 +3,8 @@ using MoShiro.Protocol;
 
 namespace MoShiro.Backend.Application;
 
-public sealed record ReportedLink(LinkHealth Health, long AtMs);
+/// <summary><paramref name="DownSinceMs"/>：控制层从什么时候起一直不是在线（Unix 毫秒）；在线时为空。重试会让控制层状态反复进出退避，所以由 Backend 连着记，不用状态自己的时间。</summary>
+public sealed record ReportedLink(LinkHealth Health, long AtMs, long? DownSinceMs = null);
 
 /// <summary>Latest per-line control and task health reported by dialing nodes. Volatile runtime state, not historical metrics.</summary>
 public sealed class LinkRuntime
@@ -20,7 +21,8 @@ public sealed class LinkRuntime
         {
             var key = (dialerId, link.ListenerId, link.IpVersion);
             current.Add(key);
-            _links[key] = new ReportedLink(link, atMs);
+            long? downSince = link.Control.State == ControlState.Up ? null : _links.TryGetValue(key, out ReportedLink? previous) && previous.DownSinceMs is { } since ? since : atMs;
+            _links[key] = new ReportedLink(link, atMs, downSince);
         }
 
         foreach ((string Dialer, string Listener, string IpVersion) key in _links.Keys.Where(k => k.Dialer == dialerId && !current.Contains(k)))
@@ -41,7 +43,7 @@ public sealed class LinkRuntime
             old.Control.RetryInMs = null;
             old.Task.Quic.State = QuicState.Idle;
             old.Task.Quic.RetryInMs = null;
-            _links[pair.Key] = new ReportedLink(old, atMs);
+            _links[pair.Key] = new ReportedLink(old, atMs, pair.Value.DownSinceMs ?? atMs);
         }
     }
 }

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);
+    }
+}