Add sections 11 (JetStream) and 12 (Clustering) with verified coverage
tables. Correct sections 2-4 where ROUTER/GATEWAY/LEAF and RS+/RS-/RMSG
were previously marked Y but are partial or stub implementations:
- ROUTER: handshake + in-memory subscription tracking only; no RMSG
- GATEWAY/LEAF: config-only stubs with no networking
- RS+/RS-/RMSG: command matrix only; no wire routing
- JetStream: 4 of ~20 API subjects implemented; RAFT is in-memory
simulation with no persistence or network transport
Add SYSTEM and ACCOUNT connection types with InternalClient,
InternalEventSystem, system event publishing, request-reply services,
and cross-account import/export support.
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.