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/EventStreamService.cs

@@ -48,6 +48,7 @@ public sealed class EventStreamService(
     /// </remarks>
     public async IAsyncEnumerable<MxEvent> StreamEventsAsync(
         StreamEventsRequest request,
+        string? callerKeyId,
         [EnumeratorCancellation] CancellationToken cancellationToken)
     {
         if (!sessionManager.TryGetSession(request.SessionId, out GatewaySession? session) || session is null)
@@ -57,6 +58,19 @@ public sealed class EventStreamService(
                 $"Session {request.SessionId} was not found.");
         }
 
+        // Owner-scoped attach (TST-02, security control): a session's event stream may be
+        // attached or reattached ONLY by the API key that opened the session. The detach-grace
+        // and fan-out retention windows are on by default, so without this check any event-scoped
+        // key that learns a session id could attach to another key's retained session and receive
+        // its replayed and live data. Ordinal comparison; null owner (session opened with no auth)
+        // matches only a null caller key.
+        if (!string.Equals(session.OwnerKeyId, callerKeyId, StringComparison.Ordinal))
+        {
+            throw new SessionManagerException(
+                SessionManagerErrorCode.PermissionDenied,
+                $"Session {request.SessionId} is owned by a different API key; event-stream attach is owner-scoped.");
+        }
+
         // No `using` here — subscriber.Dispose() is called exactly once in the finally
         // block below, which also disposes the reader. A `using` declaration would add a
         // second Dispose on the same path and double-decrement the session subscriber count.