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

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

@@ -0,0 +1,21 @@
+namespace ZB.MOM.WW.ScadaBridge.AuditLog.Central;
+
+/// <summary>
+/// Counter sink invoked by <see cref="AuditLogPurgeActor"/> every time a purge
+/// step throws (partition switch-out, per-channel override DELETE, or the pre-purge
+/// boundary enumeration). Mirrors <see cref="ICentralAuditWriteFailureCounter"/>
+/// one-for-one — same one-method contract, same NoOp default, same
+/// never-abort-the-tick invariant (the purge actor swallows the failure per
+/// boundary and continues).
+/// </summary>
+/// <remarks>
+/// A NoOp default is the safe fallback for site/test composition roots that do not
+/// wire the central health surface; <see cref="AuditCentralHealthSnapshot"/> is the
+/// production binding that routes increments into the aggregated central health
+/// snapshot's <see cref="IAuditCentralHealthSnapshot.PurgeFailures"/> read surface.
+/// </remarks>
+public interface IAuditPurgeFailureCounter
+{
+    /// <summary>Increment the central AuditLog purge-failure counter by one.</summary>
+    void Increment();
+}