server: HTTP echo + TLS reference (control-plane security measurements)

- POST /v1/echo: returns the received request head + body (base64) and the
  observed TLS parameters (version, cipher, SNI, ALPN, resumed). The client
  diffs against what it sent to detect header injection/stripping,
  transparent proxying, or TLS interception (sec.http_echo). http-echo
  added to the capability set.
- GET /v1/tls-reference: the served leaf-first DER chain + pin, so the app
  can compare an out-of-band copy against its own handshake (sec.tls_reference).
  Always available, no auth — public handshake info.
- Optional CLEARTEXT http-echo listener (ECHOLOT_HTTP_ECHO_LISTEN, default
  off) exposing only /v1/echo for the plaintext-path tampering test.

Live-smoke-tested (HTTPS echo reflected an injected header + observed
TLS1.3; cleartext variant reports tls:none); httptest unit tests added.

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

server/cmd/echolot-server/main.go

@@ -102,7 +102,7 @@ func serve(cfg *config.Config) error {
 	dp := &dataplane.Server{Sessions: sessions}
 	tcpSrv := &tcpecho.Server{}
 
-	caps := []string{"udp-probe", "delayed-echo", "connect-back"}
+	caps := []string{"udp-probe", "delayed-echo", "connect-back", "http-echo"}
 	if len(config.Addrs(cfg.TCPListen)) > 0 {
 		caps = append(caps, "tcp-echo")
 	}
@@ -110,7 +110,7 @@ func serve(cfg *config.Config) error {
 	ctl := &control.Server{
 		Store: st, Sessions: sessions, Name: cfg.Name,
 		UDPPort: mustPort(firstAddr(cfg.UDPListen)), TCPPort: mustPort(firstAddr(cfg.TCPListen)),
-		StunPort: mustPort(firstAddr(cfg.StunListen)), PinB64: pin,
+		StunPort: mustPort(firstAddr(cfg.StunListen)), PinB64: pin, CertChain: cert.Certificate,
 		DelayedEcho: dp.SendDelayedEcho,
 		TCPRecent:   func(ip string) any { return tcpSrv.RecentFor(ip) },
 	}
@@ -189,6 +189,15 @@ func serve(cfg *config.Config) error {
 		}(addr, ln)
 	}
 
+	// Optional cleartext HTTP-echo (spec §4 plaintext-path test) — only
+	// POST /v1/echo, no auth, no secrets. Off unless configured.
+	var httpEchoSrvs []*http.Server
+	for _, addr := range config.Addrs(cfg.HTTPEchoListen) {
+		hs := &http.Server{Addr: addr, Handler: ctl.EchoHandler(), ReadHeaderTimeout: 10 * time.Second}
+		httpEchoSrvs = append(httpEchoSrvs, hs)
+		go func(a string, srv *http.Server) { errCh <- fmt.Errorf("http-echo %s: %w", a, srv.ListenAndServe()) }(addr, hs)
+	}
+
 	// STUN (spec §4) — advertises stun-5780 only with ≥2 same-family addrs.
 	var stunSrv *stun.Server
 	if stunAddrs := config.Addrs(cfg.StunListen); len(stunAddrs) > 0 {
@@ -263,6 +272,9 @@ func serve(cfg *config.Config) error {
 		for _, l := range dnsTCP {
 			_ = l.Close()
 		}
+		for _, hs := range httpEchoSrvs {
+			_ = hs.Shutdown(shutCtx)
+		}
 		return nil
 	case err := <-errCh:
 		return err