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>

nodejs/server.js

@@ -0,0 +1,102 @@
+'use strict';
+
+const coap = require('coap');
+
+const PORT = Number(process.env.COAP_PORT) || 5683;
+const HOST = process.env.COAP_HOST || '0.0.0.0';
+
+// --- helpers ---------------------------------------------------------------
+
+const TEXT = 'text/plain';
+const JSON_FMT = 'application/json';
+
+/** Split "/hello?name=Bob" into ["/hello", "name=Bob"]. */
+function splitUrl(url) {
+  const i = url.indexOf('?');
+  const path = (i === -1 ? url : url.slice(0, i)).replace(/\/+$/, '') || '/';
+  return [path, i === -1 ? '' : url.slice(i + 1)];
+}
+
+/** Pull a name out of "?name=Bob" style queries. */
+function nameFromQuery(query) {
+  return new URLSearchParams(query || '').get('name');
+}
+
+/** Pull a name out of a POST body: raw text or {"name":"Bob"}. */
+function nameFromBody(buf) {
+  const body = buf.toString('utf8').trim();
+  if (!body) return null;
+  if (body.startsWith('{')) {
+    try {
+      const parsed = JSON.parse(body);
+      return typeof parsed.name === 'string' ? parsed.name.trim() : null;
+    } catch {
+      return null;
+    }
+  }
+  return body;
+}
+
+function greet(name) {
+  return `Hello, ${name && name.length ? name : 'World'}!`;
+}
+
+function send(res, code, payload, format = TEXT) {
+  res.code = code;
+  res.setOption('Content-Format', format);
+  res.end(typeof payload === 'string' ? payload : JSON.stringify(payload));
+}
+
+// --- routes ----------------------------------------------------------------
+// key: "METHOD /path"
+
+const routes = {
+  'GET /hello': (req, res) => {
+    send(res, '2.05', greet(nameFromQuery(req.query)));
+  },
+
+  'POST /hello': (req, res) => {
+    const name = nameFromBody(req.payload) || nameFromQuery(req.query);
+    if (!name) {
+      return send(res, '4.00', 'Bad Request: send a name as the payload');
+    }
+    send(res, '2.01', greet(name));
+  },
+
+  'GET /health': (_req, res) => {
+    send(res, '2.05', { status: 'ok', uptime: Math.round(process.uptime()) }, JSON_FMT);
+  },
+};
+
+// --- server ----------------------------------------------------------------
+
+const server = coap.createServer({ type: 'udp4' });
+
+server.on('request', (req, res) => {
+  const [path, query] = splitUrl(req.url);
+  req.query = query; // node-coap only gives us req.url on the server side
+  const key = `${req.method} ${path}`;
+  const handler = routes[key];
+
+  console.log(`${req.method} ${req.url} from ${req.rsinfo.address}:${req.rsinfo.port}`);
+
+  if (!handler) return send(res, '4.04', `Not Found: ${path}`);
+
+  try {
+    handler(req, res);
+  } catch (err) {
+    console.error('handler error:', err);
+    send(res, '5.00', 'Internal Server Error');
+  }
+});
+
+server.on('error', (err) => console.error('server error:', err));
+
+server.listen(PORT, HOST, () => {
+  console.log(`CoAP server (UDP) listening on coap://${HOST}:${PORT}`);
+  console.log('  GET  /hello?name=Imran');
+  console.log('  POST /hello   (payload: Imran  |  {"name":"Imran"})');
+  console.log('  GET  /health');
+});
+
+process.on('SIGINT', () => server.close(() => process.exit(0)));