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/control/httpecho_test.go

@@ -0,0 +1,63 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package control
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestHTTPEchoReflectsRequest(t *testing.T) {
+	s := &Server{}
+	req := httptest.NewRequest("POST", "/v1/echo", strings.NewReader("payload-bytes"))
+	req.Header.Set("X-Injected", "canary")
+	rr := httptest.NewRecorder()
+	s.httpEcho(rr, req)
+
+	var resp struct {
+		RequestHeadB64 string `json:"request_head_b64"`
+		BodyB64        string `json:"body_b64"`
+		BodyLen        int    `json:"body_len"`
+		Scheme         string `json:"scheme"`
+	}
+	if err := json.Unmarshal(rr.Body.Bytes(), &resp); err != nil {
+		t.Fatal(err)
+	}
+	head, _ := base64.StdEncoding.DecodeString(resp.RequestHeadB64)
+	if !strings.Contains(string(head), "X-Injected: canary") {
+		t.Fatalf("echo did not reflect the injected header:\n%s", head)
+	}
+	body, _ := base64.StdEncoding.DecodeString(resp.BodyB64)
+	if string(body) != "payload-bytes" || resp.BodyLen != 13 {
+		t.Fatalf("body mismatch: %q len=%d", body, resp.BodyLen)
+	}
+	if resp.Scheme != "http" { // httptest requests carry no TLS
+		t.Fatalf("scheme = %s, want http", resp.Scheme)
+	}
+}
+
+func TestTLSReferenceReturnsChain(t *testing.T) {
+	s := &Server{PinB64: "TESTPIN", CertChain: [][]byte{{0x30, 0x82, 0x01}, {0xAA, 0xBB}}}
+	rr := httptest.NewRecorder()
+	s.tlsReference(rr, httptest.NewRequest("GET", "/v1/tls-reference", nil))
+
+	var resp struct {
+		PinSHA256 string   `json:"pin_sha256"`
+		ChainDER  []string `json:"chain_der"`
+	}
+	if err := json.Unmarshal(rr.Body.Bytes(), &resp); err != nil {
+		t.Fatal(err)
+	}
+	if resp.PinSHA256 != "TESTPIN" || len(resp.ChainDER) != 2 {
+		t.Fatalf("bad tls-reference: %+v", resp)
+	}
+	first, _ := base64.StdEncoding.DecodeString(resp.ChainDER[0])
+	if len(first) != 3 || first[0] != 0x30 {
+		t.Fatalf("leaf DER not round-tripped: %x", first)
+	}
+}
+