fix(archreview): gateway core P0 remediation (GWC-01/02/03, TST-02, TST-12)

Interlocking changes across the gateway server (shared GatewaySession.cs /
SessionManager.cs), committed together:

- GWC-01 (Critical): alarm monitor now attaches as an internal
  (non-counted) distributor subscriber instead of a second raw drain of the
  single worker event channel; WorkerClient._events -> SingleReader with a
  claimed-once guard so a future dual-consumer regression throws loudly.
- GWC-02 (High): faulted sessions are swept in CloseExpiredLeasesAsync
  (IsFaultedReapable + FaultedReason); new FaultedGraceSeconds (default 0).
- GWC-03 (High): configurable MaxSparseArrayLength (default 1_000_000)
  enforced before allocation.
- TST-02 (High, security): StreamEvents attach now enforces the opening key
  id -> PermissionDenied on owner mismatch.
- TST-12 (Medium): CLAUDE.md retention-defaults sentence corrected.

Verified: NonWindows build clean; targeted tests 135/135 on macOS, plus
WorkerClientTests 18/18 on the Windows host.

src/ZB.MOM.WW.MxGateway.Server/Grpc/MxAccessGatewayService.cs

@@ -148,7 +148,7 @@ public sealed class MxAccessGatewayService(
         {
             requestValidator.ValidateStreamEvents(request);
             await foreach (MxEvent publicEvent in eventStreamService
-                .StreamEventsAsync(request, context.CancellationToken)
+                .StreamEventsAsync(request, identityAccessor.Current?.KeyId, context.CancellationToken)
                 .WithCancellation(context.CancellationToken)
                 .ConfigureAwait(false))
             {
@@ -931,6 +931,7 @@ public sealed class MxAccessGatewayService(
             SessionManagerErrorCode.SessionLimitExceeded => StatusCode.ResourceExhausted,
             SessionManagerErrorCode.OpenFailed => StatusCode.Unavailable,
             SessionManagerErrorCode.CloseFailed => StatusCode.Unavailable,
+            SessionManagerErrorCode.PermissionDenied => StatusCode.PermissionDenied,
             _ => StatusCode.Unavailable,
         };