refactor: stabilize legacy-core linking and header ownership

- Make include/ the canonical declarations for data_model/html_util/json_codec and convert dd3_legacy_core header copies to thin forwarders.
- Add stable public forwarders for app_context/receiver_pipeline/sender_state_machine and update refactor smoke test to stop using ../../src includes.
- Force-link dd3_legacy_core from setup() to ensure deterministic PlatformIO LDF linking across firmware envs.
- Refresh docs (README, Requirements, docs/TESTS.md) to reflect current module paths and smoke-test include strategy.

test/test_refactor_smoke/test_refactor_smoke.cpp

@@ -1,9 +1,9 @@
 #include <Arduino.h>
 #include <unity.h>
 
-#include "../../src/app_context.h"
-#include "../../src/receiver_pipeline.h"
-#include "../../src/sender_state_machine.h"
+#include "app_context.h"
+#include "receiver_pipeline.h"
+#include "sender_state_machine.h"
 #include "config.h"
 
 static void test_refactor_headers_and_types() {