mbproxy: fix the dashboard's C2/M-series review findings

Closes the on-demand-capture leak cluster from the code review. The capture's armed state was driven off SignalR's ConnectionId, which changes on every transport reconnect, so a reconnect-during-view leaked a subscriber and left the capture armed forever with no viewer. PlcSubscriptionTracker now keys on a stable per-page-load tabId, and StatusBroadcaster reconciles capture arm state from the live viewer set each push cycle — making arming single-threaded and reconnect-safe. Also fixes the TagValueCapture disarm-vs-record race, the bind-failure broadcaster/listener leak, removes dead JSON-context code, and reworks the frontend cold-start retry plus an unknown-PLC watchdog. Adds tracker / broadcaster-loop / race / wire-shape test coverage.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/tests/Mbproxy.Tests/Admin/StatusBroadcasterTests.cs

@@ -45,6 +45,8 @@ public sealed class StatusBroadcasterTests
         hostBuilder.Configuration.AddInMemoryCollection(new Dictionary<string, string?>
         {
             ["Mbproxy:AdminPort"] = "0",
+            // Fast tick so the LoopAsync test observes several cycles quickly.
+            ["Mbproxy:AdminPushIntervalMs"] = "100",
         });
         hostBuilder.Services.AddSerilog(
             new LoggerConfiguration().MinimumLevel.Fatal().CreateLogger(), dispose: false);
@@ -96,7 +98,7 @@ public sealed class StatusBroadcasterTests
     {
         await using var h = await BuildAsync();
         h.Registry.GetOrCreate("plc-x", BcdTagMap.Empty);
-        h.Tracker.Add("conn-1", "plc-x");
+        h.Tracker.SubscribePlc("conn-1", "tab-1", "plc-x");
 
         await h.Broadcaster.PushOnceAsync(TestContext.Current.CancellationToken);
 
@@ -105,16 +107,60 @@ public sealed class StatusBroadcasterTests
         push.Detail.Debug.ShouldNotBeNull();
     }
 
+    [Fact]
+    public async Task PushOnce_ReconcilesCaptureArmState_FromActiveViewers()
+    {
+        await using var h = await BuildAsync();
+        h.Registry.GetOrCreate("plc-x", BcdTagMap.Empty);
+
+        // No viewer yet — a push must leave the capture disarmed.
+        await h.Broadcaster.PushOnceAsync(TestContext.Current.CancellationToken);
+        h.Registry.TryGet("plc-x", out var capture).ShouldBeTrue();
+        capture.IsArmed.ShouldBeFalse("no detail page open — capture stays disarmed");
+
+        // A viewer opens the detail page — the next push arms the capture.
+        h.Tracker.SubscribePlc("conn-1", "tab-1", "plc-x");
+        await h.Broadcaster.PushOnceAsync(TestContext.Current.CancellationToken);
+        capture.IsArmed.ShouldBeTrue("the broadcaster reconciles the capture armed for a viewed PLC");
+
+        // The viewer leaves — the next push disarms it again.
+        h.Tracker.RemoveConnection("conn-1");
+        await h.Broadcaster.PushOnceAsync(TestContext.Current.CancellationToken);
+        capture.IsArmed.ShouldBeFalse("the broadcaster disarms a capture once its last viewer leaves");
+    }
+
     [Fact]
     public async Task StopAsync_DisarmsEveryCapture()
     {
         await using var h = await BuildAsync();
         h.Registry.GetOrCreate("plc-x", BcdTagMap.Empty);
-        h.Registry.Arm("plc-x");
+        h.Registry.ReconcileArmed(["plc-x"]);
 
         await h.Broadcaster.StopAsync();
 
         h.Registry.TryGet("plc-x", out var capture).ShouldBeTrue();
         capture.IsArmed.ShouldBeFalse();
     }
+
+    [Fact]
+    public async Task Loop_PushesRepeatedly_ThenStopsAfterStopAsync()
+    {
+        await using var h = await BuildAsync();
+
+        h.Broadcaster.Start();
+
+        // The harness runs at AdminPushIntervalMs = 100 ms; wait (generously) for the
+        // background loop to complete several cycles.
+        var deadline = DateTime.UtcNow.AddSeconds(10);
+        while (h.Sink.FleetPushes.Count < 3 && DateTime.UtcNow < deadline)
+            await Task.Delay(50, TestContext.Current.CancellationToken);
+
+        h.Sink.FleetPushes.Count.ShouldBeGreaterThanOrEqualTo(3,
+            "the background loop must push the fleet snapshot every interval");
+
+        await h.Broadcaster.StopAsync();
+        int afterStop = h.Sink.FleetPushes.Count;
+        await Task.Delay(400, TestContext.Current.CancellationToken);
+        h.Sink.FleetPushes.Count.ShouldBe(afterStop, "no pushes may occur after StopAsync");
+    }
 }