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/Sessions/SessionManagerErrorCode.cs

@@ -10,4 +10,11 @@ public enum SessionManagerErrorCode
     SessionLimitExceeded,
     OpenFailed,
     CloseFailed,
+
+    /// <summary>
+    ///     The caller is not permitted to perform the operation on this session — for example,
+    ///     attaching a <c>StreamEvents</c> stream to a session opened by a different API key.
+    ///     Maps to gRPC <c>PermissionDenied</c>.
+    /// </summary>
+    PermissionDenied,
 }