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/Configuration/EventOptions.cs

@@ -27,4 +27,14 @@ public sealed class EventOptions
     ///     bounds the buffer).
     /// </summary>
     public double ReplayRetentionSeconds { get; init; } = 300;
+
+    /// <summary>
+    ///     Gets the maximum <c>total_length</c> a sparse-array write may declare before the
+    ///     gateway rejects it with <c>InvalidArgument</c>, enforced in
+    ///     <see cref="Sessions.SparseArrayExpander"/> before the full array is materialized.
+    ///     Guards against a single write forcing a multi-GB allocation. The default of
+    ///     1,000,000 elements is far above any realistic MXAccess array write yet well below
+    ///     the frame-size ceiling.
+    /// </summary>
+    public int MaxSparseArrayLength { get; init; } = 1_000_000;
 }