test: add payload codec regression suite

lib/dd3_legacy_core/include/payload_codec.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <Arduino.h>
+
+struct BatchInput {
+  uint16_t sender_id;
+  uint16_t batch_id;
+  uint32_t t_last;
+  uint32_t present_mask;
+  uint8_t n;
+  uint16_t battery_mV;
+  uint8_t err_m;
+  uint8_t err_d;
+  uint8_t err_tx;
+  uint8_t err_last;
+  uint8_t err_rx_reject;
+  uint32_t energy_wh[30];
+  int16_t p1_w[30];
+  int16_t p2_w[30];
+  int16_t p3_w[30];
+};
+
+bool encode_batch(const BatchInput &in, uint8_t *out, size_t out_cap, size_t *out_len);
+bool decode_batch(const uint8_t *buf, size_t len, BatchInput *out);
+
+size_t uleb128_encode(uint32_t v, uint8_t *out, size_t cap);
+bool uleb128_decode(const uint8_t *in, size_t len, size_t *pos, uint32_t *v);
+
+uint32_t zigzag32(int32_t x);
+int32_t unzigzag32(uint32_t u);
+
+size_t svarint_encode(int32_t x, uint8_t *out, size_t cap);
+bool svarint_decode(const uint8_t *in, size_t len, size_t *pos, int32_t *x);
+
+#ifdef PAYLOAD_CODEC_TEST
+bool payload_codec_self_test();
+#endif