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/GatewayOptionsValidator.cs

@@ -181,6 +181,10 @@ public sealed class GatewayOptionsValidator : OptionsValidatorBase<GatewayOption
             options.DetachGraceSeconds,
             "MxGateway:Sessions:DetachGraceSeconds must be zero or greater (0 disables detach-grace retention).",
             builder);
+        AddIfNegative(
+            options.FaultedGraceSeconds,
+            "MxGateway:Sessions:FaultedGraceSeconds must be zero or greater (0 reaps a faulted session on the next sweep).",
+            builder);
         AddIfNegative(
             options.WorkerReadyWaitTimeoutMs,
             "MxGateway:Sessions:WorkerReadyWaitTimeoutMs must be greater than or equal to zero.",
@@ -214,6 +218,10 @@ public sealed class GatewayOptionsValidator : OptionsValidatorBase<GatewayOption
         builder.RequireThat(
             options.ReplayRetentionSeconds >= 0,
             "MxGateway:Events:ReplayRetentionSeconds must be greater than or equal to zero.");
+
+        builder.RequireThat(
+            options.MaxSparseArrayLength >= 1 && options.MaxSparseArrayLength <= Array.MaxLength,
+            $"MxGateway:Events:MaxSparseArrayLength must be between 1 and {Array.MaxLength}.");
     }
 
     private static void ValidateDashboard(DashboardOptions options, ValidationBuilder builder)