feat: signed releases and a host-side updater for container nodes

CI signs the image digest of each agent and probe release; the panel only
stores and forwards it. Nodes write the manifest to their data directory and
a host updater verifies the signature with a local public key, pulls by
digest, restarts the unit and rolls back when it is unhealthy. The panel can
list releases, ask a node to update and show the reported update state.

backend/tests/Monitor.TestSupport/AgentSimulator.cs

@@ -48,6 +48,13 @@ public sealed class AgentSimulator : IAsyncDisposable
 
     public ConcurrentQueue<TestRunParams> TestRuns { get; } = new();
 
+    /// <summary>面板发来的发布清单（<c>monitor.update.offer</c>）和更新请求（<c>monitor.update.apply</c>）。</summary>
+    public ConcurrentQueue<UpdateOfferParams> UpdateOffers { get; } = new();
+
+    private int _updateApplies;
+
+    public int UpdateApplies => Volatile.Read(ref _updateApplies);
+
     public ConcurrentQueue<TestId> TestCancels { get; } = new();
 
     public IReadOnlyList<RejectedItem> RejectNextState { get; set; } = [];
@@ -188,6 +195,13 @@ public sealed class AgentSimulator : IAsyncDisposable
             new TestStatusParams { TestId = test, Status = status, Phase = phase, ErrorCode = errorCode },
             MonitorJsonContext.Default.TestStatusParams));
 
+    /// <summary>像节点的更新器报告的那样，随心跳上报更新状态。</summary>
+    public Task SendUpdateStatusAsync(UpdateStatus status) =>
+        SendAsync(RpcMessage.Notification(
+            FleetMethods.Heartbeat,
+            new HeartbeatParams { Extra = JsonSerializer.SerializeToElement(new AgentHeartbeatExtra { Update = status }, MonitorJsonContext.Default.AgentHeartbeatExtra) },
+            FleetRpcContext.Default.HeartbeatParams));
+
     public async Task<RpcMessage> RequestRawAsync(string method, JsonElement? parameters = null) => await RequestAsync(method, parameters);
 
     public async Task<TaskSet> WaitForTaskSetAsync(Func<TaskSet, bool> predicate, TimeSpan? timeout = null)
@@ -389,6 +403,12 @@ public sealed class AgentSimulator : IAsyncDisposable
             case FleetMethods.Revoked:
                 Revoked = true;
                 break;
+            case MonitorMethods.UpdateOffer:
+                UpdateOffers.Enqueue(message.Params!.Value.Deserialize(MonitorJsonContext.Default.UpdateOfferParams)!);
+                break;
+            case MonitorMethods.UpdateApply:
+                Interlocked.Increment(ref _updateApplies);
+                break;
             case MonitorMethods.TestRun:
                 TestRunParams run = message.Params!.Value.Deserialize(MonitorJsonContext.Default.TestRunParams)!;
                 TestRuns.Enqueue(run);