IMU FIFO 샘플 대기 함수 추가

- 샘플 부족 방지 안전장치

src/drivers/imu/imu_i2c.h

@@ -65,6 +65,13 @@ int imu_read_temperature_cdeg(int16_t *temp_cdeg);
  */
 int imu_fifo_start(void);
 
+/**
+ * @brief Wait until the running FIFO has at least target_samples records.
+ *
+ * @return 0 if the target is reached, -ETIMEDOUT on timeout, or a negative error.
+ */
+int imu_fifo_wait_samples(uint16_t target_samples, uint32_t timeout_ms);
+
 /**
  * @brief 실행 중인 FIFO에서 최신 샘플 읽기
  *