mbproxy: close out the dashboard code-review minor findings

Resolves the remaining Minor items from the 2026-05-15 review so the
web-UI dashboard work has no open follow-ups: a real-HubConnection
end-to-end test for the SignalR feed, stable mbproxy.admin.broadcast.*
log-event names, keyboard/aria accessibility on the fleet table,
frontend JS hardening (URL-decode guard, NaN guards, shared util.js),
reconciler<->capture-registry coverage, throwing-sink and embedded-asset
tests, broadcaster polish, and a soft upper bound on AdminPushIntervalMs.

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

mbproxy/tests/Mbproxy.Tests/Configuration/ConfigReconcilerTests.cs

@@ -68,13 +68,14 @@ public sealed class ConfigReconcilerTests : IAsyncDisposable
 
     private ConfigReconciler BuildReconciler(
         IOptionsMonitor<MbproxyOptions> monitor,
-        ServiceCounters? counters = null)
+        ServiceCounters? counters = null,
+        Mbproxy.Proxy.TagCaptureRegistry? captureRegistry = null)
     {
         return new ConfigReconciler(
             monitor,
             NullLoggerFactory.Instance,
             counters ?? new ServiceCounters(),
-            new Mbproxy.Proxy.TagCaptureRegistry());
+            captureRegistry ?? new Mbproxy.Proxy.TagCaptureRegistry());
     }
 
     // The reconciler and supervisors tracked for cleanup.
@@ -347,6 +348,53 @@ public sealed class ConfigReconcilerTests : IAsyncDisposable
         foreach (var s in supervisors.Values)
             _supervisors.Add(s);
     }
+
+    // ── Test 6: Reconciler ↔ TagCaptureRegistry wiring ────────────────────────────────────
+
+    /// <summary>
+    /// The reconciler owns the tag-capture lifecycle for hot-reload: a PLC added by a
+    /// reload must get a capture entry (<c>GetOrCreate</c>), and a PLC removed by a
+    /// reload must have its capture entry dropped (<c>Remove</c>). Holds a real
+    /// <see cref="Mbproxy.Proxy.TagCaptureRegistry"/> and asserts <c>TryGet</c> tracks
+    /// the roster across an add reload and a remove reload.
+    /// </summary>
+    [Fact]
+    public async Task Apply_AddThenRemovePlc_TagCaptureRegistryTracksRoster()
+    {
+        int portA = PickFreePort();
+        int portB = PickFreePort();
+
+        var plcA    = MakePlc("A", portA);
+        var plcB    = MakePlc("B", portB);
+        var initial = MakeOptions([plcA]);
+        var withB   = MakeOptions([plcA, plcB]);
+
+        var supA = BuildSupervisor(plcA);
+        _supervisors.Add(supA);
+        await supA.StartAsync(CancellationToken.None);
+
+        var supervisors = new ConcurrentDictionary<string, PlcListenerSupervisor>(StringComparer.Ordinal)
+        {
+            ["A"] = supA,
+        };
+
+        var registry   = new Mbproxy.Proxy.TagCaptureRegistry();
+        var monitor    = new FakeOptionsMonitor(initial);
+        var reconciler = BuildReconciler(monitor, captureRegistry: registry);
+        _reconcilers.Add(reconciler);
+        reconciler.Attach(supervisors, initial);
+
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+
+        // Reload that adds PLC-B → the registry must gain a capture for B.
+        Assert.True(await reconciler.ApplyAsync(withB, cts.Token));
+        Assert.True(registry.TryGet("B", out _), "adding PLC-B must create its tag-value capture");
+        _supervisors.Add(supervisors["B"]);
+
+        // Reload that removes PLC-B → the registry must drop B's capture.
+        Assert.True(await reconciler.ApplyAsync(initial, cts.Token));
+        Assert.False(registry.TryGet("B", out _), "removing PLC-B must drop its tag-value capture");
+    }
 }
 
 /// <summary>