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.

docs/DesignDecisions.md

@@ -85,6 +85,17 @@ delivery. If the requested position precedes the oldest retained event, a
 is atomic (no gap, no duplicate). See [Sessions](./Sessions.md) for the full
 reconnect and replay protocol.
 
+Decision: event-stream attach is bound to the opening API key. Because the
+detach-grace and replay-retention windows are on by default, the reconnect
+surface is a trust boundary — a retained session outlives the stream that opened
+it. The session records the opening key id (`GatewaySession.OwnerKeyId`) and
+every `StreamEvents` attach/reattach is rejected with `PermissionDenied` unless
+the caller's key id matches the owner. Gating on the `event` scope alone was
+rejected: it would let any `event`-scoped key that learned a session id attach to
+another key's retained session and receive its replayed and live data. The check
+runs before any subscriber is attached, so a foreign key never touches the
+replay ring. Admin-scope override is deferred.
+
 ## Event Subscribers
 
 Multi-subscriber fan-out for data-side `StreamEvents` is shipped and