fix(ui): reentrancy guard on Health/AlarmSummary poll timers (arch-review S3)

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Monitoring/AlarmSummary.razor

@@ -206,6 +206,7 @@
     private bool _loading;
 
     private Timer? _refreshTimer;
+    private readonly ZB.MOM.WW.ScadaBridge.CentralUI.Services.PollGate _pollGate = new();
     private const int _autoRefreshSeconds = 15;
 
     // ── Client-side filters ──
@@ -282,10 +283,18 @@
         StopTimer();
         _refreshTimer = new Timer(_ =>
         {
+            if (!_pollGate.TryEnter()) return;
             InvokeAsync(async () =>
             {
-                await RefreshAsync();
-                StateHasChanged();
+                try
+                {
+                    await RefreshAsync();
+                    StateHasChanged();
+                }
+                finally
+                {
+                    _pollGate.Exit();
+                }
             });
         }, null, TimeSpan.FromSeconds(_autoRefreshSeconds), TimeSpan.FromSeconds(_autoRefreshSeconds));
     }

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Monitoring/Health.razor

@@ -444,6 +444,7 @@
     private IReadOnlyDictionary<string, SiteHealthState> _siteStates = new Dictionary<string, SiteHealthState>();
     private Dictionary<string, string> _siteNames = new();
     private Timer? _refreshTimer;
+    private readonly ZB.MOM.WW.ScadaBridge.CentralUI.Services.PollGate _pollGate = new();
     private int _autoRefreshSeconds = 10;
 
     // Notification Outbox headline KPIs, refreshed alongside the site states.
@@ -542,10 +543,18 @@
 
         _refreshTimer = new Timer(_ =>
         {
+            if (!_pollGate.TryEnter()) return;
             InvokeAsync(async () =>
             {
-                await RefreshNow();
-                StateHasChanged();
+                try
+                {
+                    await RefreshNow();
+                    StateHasChanged();
+                }
+                finally
+                {
+                    _pollGate.Exit();
+                }
             });
         }, null, TimeSpan.FromSeconds(_autoRefreshSeconds), TimeSpan.FromSeconds(_autoRefreshSeconds));
     }

src/ZB.MOM.WW.ScadaBridge.CentralUI/Services/PollGate.cs

@@ -0,0 +1,11 @@
+namespace ZB.MOM.WW.ScadaBridge.CentralUI.Services;
+
+/// <summary>Reentrancy guard for poll timers: a fixed-period Timer whose refresh outlives
+/// the period stacks overlapping fan-outs against an already-degraded site (arch-review S3).
+/// Callers TryEnter at tick start and skip the tick when the previous refresh is in flight.</summary>
+public sealed class PollGate
+{
+    private int _inFlight;
+    public bool TryEnter() => Interlocked.CompareExchange(ref _inFlight, 1, 0) == 0;
+    public void Exit() => Volatile.Write(ref _inFlight, 0);
+}

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/PollGateTests.cs

@@ -0,0 +1,17 @@
+using ZB.MOM.WW.ScadaBridge.CentralUI.Services;
+using Xunit;
+
+namespace ZB.MOM.WW.ScadaBridge.CentralUI.Tests;
+
+public class PollGateTests
+{
+    [Fact]
+    public void TryEnter_SecondCallWhileHeld_ReturnsFalse()
+    {
+        var gate = new PollGate();
+        Assert.True(gate.TryEnter());
+        Assert.False(gate.TryEnter());
+        gate.Exit();
+        Assert.True(gate.TryEnter());
+    }
+}