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/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);
+}