feat(audit): MxGateway IAuditActorAccessor + dashboard audit Actor = operator principal (keyId→Target) (Phase 3)

Introduce IAuditActorAccessor seam + HttpAuditActorAccessor impl (reads ZbClaimTypes.Username
from IHttpContextAccessor; falls back to Identity.Name / ZbClaimTypes.Name; null when
unauthenticated). Register in DI via DashboardServiceCollectionExtensions.

Wire DashboardApiKeyManagementService: WriteDashboardAuditAsync now accepts the ClaimsPrincipal
user already in scope at each call site; ResolveOperatorActor extracts ZbClaimTypes.Username
(preferred) or Identity.Name. All four dashboard-* events now emit Actor = LDAP operator
username and Target = managed keyId, fixing the semantic gap where both fields held the keyId.

ConstraintEnforcer (gRPC / API-key actor) and CanonicalForwardingApiKeyAuditStore (CLI /
"system"/"cli" fallback) are unchanged.

Tests: DashboardApiKeyManagementServiceTests updated — CreateAuthorizedUser adds ZbClaimTypes.Username
("alice"), all dashboard-* audit assertions updated to Actor = "alice" / Target = "operator01";
new CreateAsync_AuthorizedUser_CanonicalAuditEventHasOperatorAsActorAndKeyIdAsTarget verifies the
canonical AuditEvent directly. New HttpAuditActorAccessorTests (4 cases: username claim, Identity.Name
fallback, unauthenticated → null, no context → null). ConstraintEnforcer tests still assert API-key/anonymous actor.

src/ZB.MOM.WW.MxGateway.Server/Dashboard/DashboardServiceCollectionExtensions.cs

@@ -6,6 +6,7 @@ using Microsoft.Extensions.Options;
 using ZB.MOM.WW.Auth.Abstractions.Roles;
 using ZB.MOM.WW.Auth.AspNetCore;
 using ZB.MOM.WW.MxGateway.Server.Configuration;
+using ZB.MOM.WW.MxGateway.Server.Security.Audit;
 
 namespace ZB.MOM.WW.MxGateway.Server.Dashboard;
 
@@ -47,6 +48,7 @@ public static class DashboardServiceCollectionExtensions
         services.AddHostedService<Hubs.DashboardSnapshotPublisher>();
         services.AddHostedService<Hubs.AlarmsHubPublisher>();
         services.AddHttpContextAccessor();
+        services.AddSingleton<IAuditActorAccessor, HttpAuditActorAccessor>();
         services.AddAntiforgery();
         services.AddCascadingAuthenticationState();
         services.AddRazorComponents()