refactor: move html_util into legacy core library

lib/dd3_legacy_core/include/dd3_legacy_core.h

@@ -0,0 +1,4 @@
+#pragma once
+
+// Include this header in legacy Unity tests to force-link dd3_legacy_core.
+void dd3_legacy_core_force_link();

lib/dd3_legacy_core/include/html_util.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include <Arduino.h>
+
+String html_escape(const String &input);
+String url_encode_component(const String &input);
+bool sanitize_device_id(const String &input, String &out_device_id);

lib/dd3_legacy_core/src/dd3_legacy_core.cpp

@@ -0,0 +1,3 @@
+#include "dd3_legacy_core.h"
+
+void dd3_legacy_core_force_link() {}

test/test_html_escape/test_html_escape.cpp

@@ -1,5 +1,6 @@
 #include <Arduino.h>
 #include <unity.h>
+#include "dd3_legacy_core.h"
 #include "html_util.h"
 
 static void test_html_escape_basic() {
@@ -121,6 +122,7 @@ static void test_sanitize_device_id_rejects_invalid() {
 }
 
 void setup() {
+  dd3_legacy_core_force_link();
   UNITY_BEGIN();
   RUN_TEST(test_html_escape_basic);
   RUN_TEST(test_html_escape_adversarial);