feat(audit): AddZbAudit DI extension with safe defaults

TryAdd registers NullAuditRedactor + NoOpAuditWriter so consumer
registrations win; symmetric override tests for both writer and redactor.

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

@@ -0,0 +1,21 @@
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+
+namespace ZB.MOM.WW.Audit;
+
+/// <summary>DI helpers for ZB.MOM.WW.Audit.</summary>
+public static class AuditServiceCollectionExtensions
+{
+    /// <summary>
+    /// Registers safe defaults — <see cref="NullAuditRedactor"/> and <see cref="NoOpAuditWriter"/> —
+    /// using TryAdd so a consumer that has already registered a real writer/redactor wins. Consumers
+    /// compose <see cref="RedactingAuditWriter"/>/<see cref="CompositeAuditWriter"/> around their own sink.
+    /// </summary>
+    public static IServiceCollection AddZbAudit(this IServiceCollection services)
+    {
+        ArgumentNullException.ThrowIfNull(services);
+        services.TryAddSingleton<IAuditRedactor>(NullAuditRedactor.Instance);
+        services.TryAddSingleton<IAuditWriter>(NoOpAuditWriter.Instance);
+        return services;
+    }
+}

ZB.MOM.WW.Audit/tests/ZB.MOM.WW.Audit.Tests/AuditServiceCollectionExtensionsTests.cs

@@ -0,0 +1,32 @@
+using Microsoft.Extensions.DependencyInjection;
+
+namespace ZB.MOM.WW.Audit.Tests;
+
+public class AuditServiceCollectionExtensionsTests
+{
+    [Fact]
+    public void Registers_null_redactor_and_noop_writer_by_default()
+    {
+        var sp = new ServiceCollection().AddZbAudit().BuildServiceProvider();
+        Assert.IsType<NullAuditRedactor>(sp.GetRequiredService<IAuditRedactor>());
+        Assert.IsType<NoOpAuditWriter>(sp.GetRequiredService<IAuditWriter>());
+    }
+
+    [Fact]
+    public void Does_not_override_a_preregistered_writer()
+    {
+        var services = new ServiceCollection();
+        services.AddSingleton<IAuditWriter>(new CompositeAuditWriter(System.Array.Empty<IAuditWriter>()));
+        var sp = services.AddZbAudit().BuildServiceProvider();
+        Assert.IsType<CompositeAuditWriter>(sp.GetRequiredService<IAuditWriter>());
+    }
+
+    [Fact]
+    public void Does_not_override_a_preregistered_redactor()
+    {
+        var services = new ServiceCollection();
+        services.AddSingleton<IAuditRedactor>(new TruncatingAuditRedactor());
+        var sp = services.AddZbAudit().BuildServiceProvider();
+        Assert.IsType<TruncatingAuditRedactor>(sp.GetRequiredService<IAuditRedactor>());
+    }
+}