feat(protocol): add packet protocol

firmware/App/Inc/protocol.h

@@ -0,0 +1,40 @@
+#ifndef PROTOCOL_H
+#define PROTOCOL_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+// Protocol Constants
+#define PROTOCOL_SOF 0xAA
+#define PACKET_TYPE_AUDIO 0x02
+#define PACKET_LEN_V1 0x08  // Payload length (excluding SOF, TYPE, LEN, CRC)
+#define PACKET_TOTAL_SIZE 12
+
+// CRC8-ATM Constants
+#define CRC8_POLY 0x07
+#define CRC8_INIT 0x00
+
+/**
+ * @brief  Calculates CRC-8/ATM over the data buffer.
+ *         Polynomial: x^8 + x^2 + x + 1 (0x07)
+ *         Init: 0x00, RefIn: false, RefOut: false, XorOut: 0x00
+ * @param  data Pointer to data buffer
+ * @param  len  Length of data
+ * @return Calculated CRC8
+ */
+uint8_t crc8_atm(const uint8_t *data, size_t len);
+
+/**
+ * @brief Encodes the audio metric packet into the wire format.
+ * @param buf Output buffer (must be at least 12 bytes)
+ * @param timestamp_ms Timestamp in milliseconds
+ * @param rms_dbfs RMS value in dBFS (float)
+ * @param freq_hz Peak frequency in Hz (float)
+ */
+void protocol_pack_v1(
+    uint8_t *buf,
+    uint32_t timestamp_ms,
+    float rms_dbfs,
+    float freq_hz);
+
+#endif  // PROTOCOL_H