Covers 6 implementation layers: ClientKind enum + INatsClient interface,
event infrastructure with Channel<T>, system event publishing, request-reply
monitoring services, import/export model with ACCOUNT client, and response
routing with latency tracking.
Reconcile close reason tracking: feature branch's MarkClosed() and
ShouldSkipFlush/FlushAndCloseAsync now use main's ClientClosedReason
enum. ClosedState enum retained for forward compatibility.