feat(firmware): add IMU data structures and UART v2 parser

- Add SwitchImuSample struct and IMU fields to SwitchInputState
- Add fill_imu_report_data() to pack samples into imuData[36]
- Rewrite switch_pro_apply_uart_packet() for v2 with checksum validation
- Enlarge poll_uart_frames() buffer to 64 bytes for variable-length frames
- Clear imu_sample_count after each USB report send (prevent stale IMU)

switch_pro_driver.h

@@ -10,6 +10,15 @@
 #include <stdint.h>
 #include "switch_pro_descriptors.h"
 
+typedef struct {
+    int16_t accel_x;
+    int16_t accel_y;
+    int16_t accel_z;
+    int16_t gyro_x;
+    int16_t gyro_y;
+    int16_t gyro_z;
+} SwitchImuSample;
+
 typedef struct {
     bool dpad_up;
     bool dpad_down;
@@ -35,6 +44,9 @@ typedef struct {
     uint16_t ly;
     uint16_t rx;
     uint16_t ry;
+
+    uint8_t imu_sample_count;     // 0-3
+    SwitchImuSample imu_samples[3];
 } SwitchInputState;
 
 // Initialize USB state and calibration before entering the main loop.