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.

src/main.cpp

@@ -3,6 +3,7 @@
 #include "app_context.h"
 #include "config.h"
 #include "data_model.h"
+#include "dd3_legacy_core.h"
 #include "display_ui.h"
 #include "lora_transport.h"
 #include "mqtt_client.h"
@@ -65,6 +66,7 @@ static void watchdog_kick() {}
 void setup() {
   Serial.begin(115200);
   delay(200);
+  dd3_legacy_core_force_link();
 
 #ifdef PAYLOAD_CODEC_TEST
   payload_codec_self_test();