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.

lib/dd3_legacy_core/include/data_model.h

@@ -1,60 +1,3 @@
 #pragma once
 
-#include <Arduino.h>
-
-enum class FaultType : uint8_t {
-  None = 0,
-  MeterRead = 1,
-  Decode = 2,
-  LoraTx = 3
-};
-
-enum class RxRejectReason : uint8_t {
-  None = 0,
-  CrcFail = 1,
-  InvalidMsgKind = 2,
-  LengthMismatch = 3,
-  DeviceIdMismatch = 4,
-  BatchIdMismatch = 5,
-  UnknownSender = 6
-};
-
-struct FaultCounters {
-  uint32_t meter_read_fail;
-  uint32_t decode_fail;
-  uint32_t lora_tx_fail;
-};
-
-struct MeterData {
-  uint32_t ts_utc;
-  uint32_t meter_seconds;
-  uint16_t short_id;
-  char device_id[16];
-  float energy_total_kwh;
-  float phase_power_w[3];
-  float total_power_w;
-  float battery_voltage_v;
-  uint8_t battery_percent;
-  bool meter_seconds_valid;
-  bool valid;
-  int16_t link_rssi_dbm;
-  float link_snr_db;
-  bool link_valid;
-  uint32_t err_meter_read;
-  uint32_t err_decode;
-  uint32_t err_lora_tx;
-  FaultType last_error;
-  uint8_t rx_reject_reason;
-};
-
-struct SenderStatus {
-  MeterData last_data;
-  uint32_t last_update_ts_utc;
-  uint32_t rx_batches_total;
-  uint32_t rx_batches_duplicate;
-  uint32_t rx_last_duplicate_ts_utc;
-  bool has_data;
-};
-
-void init_device_ids(uint16_t &short_id, char *device_id, size_t device_id_len);
-const char *rx_reject_reason_text(RxRejectReason reason);
+#include "../../../include/data_model.h"

lib/dd3_legacy_core/include/html_util.h

@@ -1,7 +1,3 @@
 #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);
+#include "../../../include/html_util.h"

lib/dd3_legacy_core/include/json_codec.h

@@ -1,6 +1,3 @@
 #pragma once
 
-#include <Arduino.h>
-#include "data_model.h"
-
-bool meterDataToJson(const MeterData &data, String &out_json);
+#include "../../../include/json_codec.h"