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/internal/config/config.go

@@ -29,6 +29,9 @@ type Config struct {
 	StunListen string // ECHOLOT_STUN_LISTEN / --stun-listen (spec default 3478; empty disables)
 	DNSListen  string // ECHOLOT_DNS_LISTEN / --dns-listen (canary zone; empty disables)
 	CanaryZone string // ECHOLOT_CANARY_ZONE / --canary-zone (e.g. c.echo-lot.app)
+	// Optional cleartext HTTP-echo listener (spec §4 plaintext-path test).
+	// Default empty = off; it exposes only POST /v1/echo, no auth, no secrets.
+	HTTPEchoListen string // ECHOLOT_HTTP_ECHO_LISTEN / --http-echo-listen
 
 	// Admin UI / health listener (spec §7: localhost-only by default)
 	AdminListen string // ECHOLOT_ADMIN_LISTEN / --admin-listen
@@ -71,6 +74,7 @@ func Load(args []string) (*Config, *Actions, error) {
 	fs.StringVar(&c.StunListen, "stun-listen", envOr("STUN_LISTEN", ":3478"), "STUN listen address(es), comma-separated; empty disables (spec §4)")
 	fs.StringVar(&c.DNSListen, "dns-listen", envOr("DNS_LISTEN", ""), "canary-DNS listen address(es) udp+tcp/53, comma-separated; empty disables (spec §6.1)")
 	fs.StringVar(&c.CanaryZone, "canary-zone", envOr("CANARY_ZONE", ""), "authoritative canary zone, e.g. c.echo-lot.app")
+	fs.StringVar(&c.HTTPEchoListen, "http-echo-listen", envOr("HTTP_ECHO_LISTEN", ""), "optional CLEARTEXT http-echo listen address(es); empty disables (spec §4)")
 	fs.StringVar(&c.AdminListen, "admin-listen", envOr("ADMIN_LISTEN", "127.0.0.1:8444"), "admin/health listen address (keep localhost)")
 	fs.StringVar(&c.StateDir, "state-dir", envOr("STATE_DIR", defaultStateDir()), "state directory (device store, generated TLS)")
 	fs.StringVar(&c.Name, "name", envOr("NAME", "echolot"), "server profile name")