feat(protocol): add packet protocol

firmware/App/Src/protocol.c

@@ -0,0 +1,51 @@
+#include "protocol.h"
+#include <math.h>
+
+uint8_t crc8_atm(const uint8_t *data, size_t len) {
+    uint8_t crc = CRC8_INIT;
+    for (size_t i = 0; i < len; i++) {
+        crc ^= data[i];
+        for (uint8_t j = 0; j < 8; j++) {
+            if (crc & 0x80) {
+                crc = (crc << 1) ^ CRC8_POLY;
+            } else {
+                crc <<= 1;
+            }
+        }
+    }
+    return crc;
+}
+
+void protocol_pack_v1(
+    uint8_t *buf,
+    uint32_t timestamp_ms,
+    float rms_dbfs,
+    float freq_hz) {
+    // Header
+    buf[0] = PROTOCOL_SOF;
+    buf[1] = PACKET_TYPE_AUDIO;
+    buf[2] = PACKET_LEN_V1;
+
+    // Payload: Timestamp (4 bytes, Little Endian)
+    buf[3] = (uint8_t)(timestamp_ms & 0xFF);
+    buf[4] = (uint8_t)((timestamp_ms >> 8) & 0xFF);
+    buf[5] = (uint8_t)((timestamp_ms >> 16) & 0xFF);
+    buf[6] = (uint8_t)((timestamp_ms >> 24) & 0xFF);
+
+    // Payload: RMS_DB (2 bytes, Little Endian, x10, int16)
+    // Range check implicit by int16 cast, but clamping is safer
+    // Spec: -40..80 dB -> -400..800
+    // Note: Since DSP returns dBFS (negative), we just send it as is.
+    // E.g. -60.5 dB -> -605.
+    int16_t rms_fixed = (int16_t)(rms_dbfs * 10.0f);
+    buf[7] = (uint8_t)(rms_fixed & 0xFF);
+    buf[8] = (uint8_t)((rms_fixed >> 8) & 0xFF);
+
+    // Payload: FREQ_HZ (2 bytes, Little Endian, uint16)
+    uint16_t freq_fixed = (uint16_t)freq_hz;
+    buf[9] = (uint8_t)(freq_fixed & 0xFF);
+    buf[10] = (uint8_t)((freq_fixed >> 8) & 0xFF);
+
+    // CRC8 (Calculated over bytes 1..10: TYPE, LEN, Payload)
+    buf[11] = crc8_atm(&buf[1], 10);
+}