Portable non-blocking core with per-platform transports

The goal of nats-cljc is write-once-run-both: the same .cljc consumer code compiles and runs unchanged on the JVM, the browser, and Node. A browser can reach NATS only over WebSocket, and ClojureScript cannot block a thread. We therefore make the core API non-blocking on every platform — no blocking call leaks into the portable surface — and fix one transport per platform: JVM ⇒ TCP (via jnats), all ClojureScript ⇒ WebSocket. Any NATS server reachable from a ClojureScript platform must have its websocket listener enabled.

Considered options

• Expose the JVM's native blocking API and a separate async CLJS API — rejected: defeats write-once-run-both; the two surfaces would diverge.
• Block on the JVM, async on CLJS, behind one name — rejected: not portable, since a .cljc consumer can't block on CLJS.

Consequences

• JVM users do not get blocking-by-default. A thin blocking convenience layer may be offered in .clj-only namespaces, built on top of the portable core, for callers who want it.
• Every NATS deployment that serves ClojureScript clients must enable WebSocket. This is an operational requirement we document, not something the library can paper over.
• Node-CLJS rides the same WebSocket path as the browser (raw TCP on Node is out of scope for now), keeping the ClojureScript side to a single transport code path.