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.Tests/Security/Authorization/GatewayGrpcAuthorizationInterceptorTests.cs

@@ -477,6 +477,14 @@ public sealed class GatewayGrpcAuthorizationInterceptorTests
             return AsyncEnumerable.Empty<WorkerEvent>();
         }
 
+        /// <inheritdoc />
+        public IAsyncEnumerable<MxEvent> ReadAlarmEventsAsync(
+            string sessionId,
+            CancellationToken cancellationToken)
+        {
+            return AsyncEnumerable.Empty<MxEvent>();
+        }
+
         /// <inheritdoc />
         public Task<SessionCloseResult> CloseSessionAsync(
             string sessionId,
@@ -515,6 +523,7 @@ public sealed class GatewayGrpcAuthorizationInterceptorTests
         /// <inheritdoc />
         public async IAsyncEnumerable<MxEvent> StreamEventsAsync(
             StreamEventsRequest request,
+            string? callerKeyId,
             [EnumeratorCancellation] CancellationToken cancellationToken)
         {
             await Task.CompletedTask;