Add HTTP bridge so the CoAP server can be checked in a browser

Browsers speak HTTP over TCP and cannot open coap:// URLs, which made the
server awkward to try without the CLI client. The bridge accepts an ordinary
HTTP request, reissues it as CoAP over UDP, and maps the response code back
onto an HTTP status (the raw code is also returned in X-Coap-Code).

It serves a test console at / with a name field and a button per route, so
GET, POST, 4.04 and 4.05 can all be exercised by clicking.

The CoAP request logic moves from src/bin/client.rs into src/lib.rs so the
client and the bridge share one implementation.

The bridge is a development convenience: a small HTTP/1.1 subset with no
keep-alive, chunked bodies or TLS.

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

Cargo.lock

@@ -8,6 +8,12 @@ version = "1.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f2032f911046de80f0a198e0901378627c33f59ea0ac00e363d481118bd70a53"
 
+[[package]]
+name = "bytes"
+version = "1.12.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8ae3f5d315924270530207e2a68396c3cc547f6dca3fbdca317cfb1a51edb593"
+
 [[package]]
 name = "coap-hello-udp"
 version = "0.1.0"
@@ -145,6 +151,7 @@ version = "1.53.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "202caea871b69668250d242070849eb495be178ed697a3e98aebce5bc81a0bed"
 dependencies = [
+ "bytes",
  "libc",
  "mio",
  "pin-project-lite",