Add CoAP-over-UDP hello server in Rust

A minimal CoAP (RFC 7252) server built on coap-lite for message encoding
and a tokio UdpSocket for transport. The socket, read loop and routing are
kept in src/main.rs rather than behind a framework.

Routes:
  GET  /hello[?name=X]  -> 2.05 "Hello, X!" (defaults to World)
  POST /hello           -> 2.01, name from a raw text or JSON body
  GET  /health          -> 2.05 status + uptime

Unknown paths return 4.04, known paths with the wrong method 4.05.

Includes a CLI client (src/bin/client.rs) for smoke tests and unit tests
covering the greeting, payload parsing and query decoding. Verified against
node-coap as an independent implementation to confirm wire compatibility.

The earlier Node.js prototype is kept under nodejs/ for reference.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,2 @@
+target/
+nodejs/node_modules/