feat(audit): OtOpcUa IAuditActorAccessor seam + HTTP impl (audit Actor from Auth principal) (Phase 3)

Introduces the IAuditActorAccessor seam and HttpAuditActorAccessor impl so the
ZB.MOM.WW.Audit.AuditEvent Actor field can be sourced from the authenticated Blazor
cookie principal (ZbClaimTypes.Username) when structured emitters are added. Adds the
AuditActor.Resolve static helper (accessor value → SystemFallback/"system") as the
canonical pattern for future emit sites. Wires DI in AddOtOpcUaAuth (TryAddScoped) with
AddHttpContextAccessor(). The structured AuditEvent path remains DORMANT — no live emit
sites exist; seam is forward-looking. SP-based audit path left untouched. 9 new unit
tests all green; Security (54) and ControlPlane (45) test suites fully pass.

src/Server/ZB.MOM.WW.OtOpcUa.Security/Audit/IAuditActorAccessor.cs

@@ -0,0 +1,30 @@
+namespace ZB.MOM.WW.OtOpcUa.Security.Audit;
+
+/// <summary>
+/// Resolves the current HTTP principal's actor string for inclusion in a canonical
+/// <c>ZB.MOM.WW.Audit.AuditEvent</c> as the <c>Actor</c> field.
+/// </summary>
+/// <remarks>
+/// The seam abstracts the identity source so that:
+/// <list type="bullet">
+///   <item>production code uses <see cref="HttpAuditActorAccessor"/> (reads the
+///   authenticated Blazor cookie principal from <c>IHttpContextAccessor</c>); and</item>
+///   <item>unit tests or non-HTTP contexts can substitute a stub or return
+///   <see langword="null"/> (which triggers the <c>"system"</c> fallback in
+///   <see cref="AuditActor.Resolve"/>).</item>
+/// </list>
+/// <para>
+/// <b>Note:</b> OtOpcUa has no live structured <c>AuditEvent</c> emit sites as of Phase 3
+/// (all production audit flows through the bespoke stored-procedure path). This seam is
+/// forward-looking — wired and tested so that future emit sites can call
+/// <see cref="AuditActor.Resolve"/> and get the Auth principal automatically.
+/// </para>
+/// </remarks>
+public interface IAuditActorAccessor
+{
+    /// <summary>
+    /// Returns the authenticated principal's actor string, or <see langword="null"/> when
+    /// there is no current HTTP context or the user is not authenticated.
+    /// </summary>
+    string? CurrentActor { get; }
+}