feat(audit-log): surface partition-purge failure as health event + counter

src/ZB.MOM.WW.ScadaBridge.AuditLog/Central/NoOpAuditPurgeFailureCounter.cs

@@ -0,0 +1,16 @@
+namespace ZB.MOM.WW.ScadaBridge.AuditLog.Central;
+
+/// <summary>
+/// Default <see cref="IAuditPurgeFailureCounter"/> binding used when the central
+/// health surface (<see cref="AuditCentralHealthSnapshot"/>) has not been wired
+/// (test composition roots, site-only hosts). Drops every increment on the floor.
+/// Mirrors <see cref="NoOpCentralAuditWriteFailureCounter"/>.
+/// </summary>
+public sealed class NoOpAuditPurgeFailureCounter : IAuditPurgeFailureCounter
+{
+    /// <inheritdoc/>
+    public void Increment()
+    {
+        // intentional no-op
+    }
+}