feat(audit): AuditEvent record + AuditOutcome + writer/redactor seams

Includes equality-as-normalized-instant remarks on OccurredAtUtc and a
same-instant/different-offset equality regression test (code-review follow-up).

ZB.MOM.WW.Audit/src/ZB.MOM.WW.Audit/IAuditWriter.cs

@@ -0,0 +1,12 @@
+namespace ZB.MOM.WW.Audit;
+
+/// <summary>
+/// Best-effort sink for <see cref="AuditEvent"/>s. Implementations MUST swallow/log internal
+/// failures rather than propagating them — a failed audit write must never abort the
+/// user-facing action that produced it.
+/// </summary>
+public interface IAuditWriter
+{
+    /// <summary>Persist an audit event. Best-effort; must not throw to the caller.</summary>
+    Task WriteAsync(AuditEvent evt, CancellationToken ct = default);
+}