feat(health): CentralAuditWriteFailures + AuditCentralHealthSnapshot (#23 M6)

src/ScadaLink.AuditLog/Central/ICentralAuditWriteFailureCounter.cs

@@ -0,0 +1,23 @@
+namespace ScadaLink.AuditLog.Central;
+
+/// <summary>
+/// Audit Log (#23) M6 Bundle E (T8) counter sink invoked by central-side audit
+/// writers (<see cref="CentralAuditWriter"/>, <see cref="AuditLogIngestActor"/>)
+/// every time a repository <c>InsertIfNotExistsAsync</c> throws. Mirrors the
+/// site-side <see cref="ScadaLink.AuditLog.Site.IAuditWriteFailureCounter"/>
+/// shape one-for-one — same one-method contract, same NoOp default, same
+/// must-never-abort-the-user-facing-action invariant.
+/// </summary>
+/// <remarks>
+/// Audit-write failures NEVER abort the user-facing action (alog.md §13) —
+/// the writer swallows the exception and surfaces the failure via this counter
+/// instead. A NoOp default is the correct safe fallback while the central
+/// health surface is being wired in; <see cref="AuditCentralHealthSnapshot"/>
+/// is the production binding that routes increments into the aggregated
+/// central health snapshot consumed by future M7+ pages.
+/// </remarks>
+public interface ICentralAuditWriteFailureCounter
+{
+    /// <summary>Increment the central audit-write failure counter by one.</summary>
+    void Increment();
+}