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.

include/app_context.h

@@ -0,0 +1,3 @@
+#pragma once
+
+#include "../src/app_context.h"

include/receiver_pipeline.h

@@ -0,0 +1,3 @@
+#pragma once
+
+#include "../src/receiver_pipeline.h"

include/sender_state_machine.h

@@ -0,0 +1,3 @@
+#pragma once
+
+#include "../src/sender_state_machine.h"