fix(usb): resolve USB CDC TX failure caused by three independent bugs

Bug 1 (PRIMARY — DCache/USB coherency):
SCB_DisableDCache() was buried inside icm42688_init(), called ~3.5s
after USB starts. STM32F7 DCache/USB coherency issue: when DCache is
on (enabled by SystemInit()), CPU writes to TX buffers stay in cache
and the USB hardware reads stale SRAM data. Moved SCB_DisableDCache()
to main() before HAL_Init(), ensuring coherency for all USB transfers.

Bug 2 (USB TX corruption):
CDC_Transmit() passed the caller's stack-allocated buf pointer directly
to the USB stack. The USB TXFE interrupt fires asynchronously; by then
the stack buffer may have been modified by the next loop iteration.
CDC_Transmit() now copies into the static UserTxBuffer before handing
off to the USB hardware, ensuring the buffer is stable for the transfer.

Bug 3 (IMU type mismatch → wrong data to balance):
main.c called icm42688_init()/icm42688_read() directly, passing
icm42688_data_t* (raw int16 ax/ay/az/gx/gy/gz) to balance_update()
which expects IMUData* (float pitch/pitch_rate from complementary
filter). Type mismatch produced garbage balance values. Fixed by using
mpu6000_init()/mpu6000_read() which wraps icm42688 + sensor fusion.
Telemetry updated to report fused pitch/rate instead of raw ADC counts.

Also fix icm42688_init() returning 0 on who==0 (no SPI response),
which falsely indicated IMU success.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/USB_CDC/src/usbd_cdc_if.c

@@ -121,7 +121,6 @@ static int8_t CDC_Receive(uint8_t *buf, uint32_t *len) {
 USBD_CDC_ItfTypeDef USBD_CDC_fops = { CDC_Init, CDC_DeInit, CDC_Control, CDC_Receive };
 
 uint8_t CDC_Transmit(uint8_t *buf, uint16_t len) {
-
     USBD_CDC_HandleTypeDef *hcdc = (USBD_CDC_HandleTypeDef *)hUsbDevice.pClassData;
     if (hcdc == NULL) return USBD_FAIL;
     if (hcdc->TxState != 0) {
@@ -130,6 +129,12 @@ uint8_t CDC_Transmit(uint8_t *buf, uint16_t len) {
         if (++busy_count > 100) { hcdc->TxState = 0; busy_count = 0; }
         return USBD_BUSY;
     }
-    USBD_CDC_SetTxBuffer(&hUsbDevice, buf, len);
+    /* Always copy into the static UserTxBuffer so the USB hardware reads
+     * from a known fixed SRAM address — never from the caller's stack.
+     * The USB TXFE IRQ fires asynchronously; a stack buffer could be
+     * overwritten by the time the FIFO is loaded. */
+    if (len > sizeof(UserTxBuffer)) len = sizeof(UserTxBuffer);
+    memcpy(UserTxBuffer, buf, len);
+    USBD_CDC_SetTxBuffer(&hUsbDevice, UserTxBuffer, len);
     return USBD_CDC_TransmitPacket(&hUsbDevice);
 }

src/icm42688.c

@@ -104,7 +104,8 @@ int icm42688_init(void) {
     HAL_GPIO_Init(MPU_CS_PORT, &gpio);
     cs_high();
 
-    SCB_DisableDCache();  /* Avoid SPI DCache issues on F7 */
+    /* DCache is disabled at startup in main.c before USB/peripherals init.
+     * No SCB_DisableDCache() here — calling it after USB starts corrupts USB state. */
 
     hspi1.Instance = SPI1;
     hspi1.Init.Mode = SPI_MODE_MASTER;
@@ -137,7 +138,8 @@ int icm42688_init(void) {
         imu_type = 2;
         ret = -99; /* TODO: ICM init */
     } else {
-        ret = -who;
+        /* who==0 means no SPI response — must not return 0 (false success) */
+        ret = (who != 0) ? -(int)who : -128;
     }
 
     /* Speed up SPI for reads */

src/main.c

@@ -3,8 +3,7 @@
 #include "usbd_cdc.h"
 #include "usbd_cdc_if.h"
 #include "usbd_desc.h"
-#include "icm42688.h"
-#include "bmp280.h"
+#include "mpu6000.h"
 #include "balance.h"
 #include "hoverboard.h"
 #include "config.h"
@@ -54,6 +53,7 @@ void SysTick_Handler(void) { HAL_IncTick(); }
 
 int main(void) {
     SCB_EnableICache();
+    SCB_DisableDCache();  /* Must be off before USB starts — STM32F7 DCache/USB coherency */
     checkForBootloader(); /* Check RTC magic BEFORE HAL_Init — must be first thing */
     HAL_Init();
     SystemClock_Config();
@@ -67,8 +67,8 @@ int main(void) {
     status_init();
     HAL_Delay(3000);  /* Wait for USB host to enumerate */
 
-    /* Init IMU */
-    int imu_ret = icm42688_init();
+    /* Init IMU (MPU6000 via SPI1 — mpu6000.c wraps icm42688 + complementary filter) */
+    int imu_ret = mpu6000_init() ? 0 : -1;
 
     /* Init hoverboard ESC UART */
     hoverboard_init();
@@ -80,14 +80,14 @@ int main(void) {
     char buf[256];
     int len;
 
-    icm42688_data_t imu;
+    IMUData imu;
     uint32_t send_tick = 0;
     uint32_t balance_tick = 0;
     uint32_t esc_tick = 0;
     const float dt = 1.0f / PID_LOOP_HZ;  /* 1ms at 1kHz */
 
     while (1) {
-        if (imu_ret == 0) icm42688_read(&imu);
+        if (imu_ret == 0) mpu6000_read(&imu);
 
         uint32_t now = HAL_GetTick();
 
@@ -123,10 +123,9 @@ int main(void) {
             send_tick = now;
             if (imu_ret == 0) {
                 len = snprintf(buf, sizeof(buf),
-                    "{\"ax\":%d,\"ay\":%d,\"az\":%d,\"gx\":%d,\"gy\":%d,\"gz\":%d,"
-                    "\"p\":%d,\"m\":%d,\"s\":%d}\n",
-                    imu.ax, imu.ay, imu.az, imu.gx, imu.gy, imu.gz,
-                    (int)(bal.pitch_deg * 10),   /* pitch x10 for precision without %f */
+                    "{\"p\":%d,\"r\":%d,\"m\":%d,\"s\":%d}\n",
+                    (int)(imu.pitch * 10),       /* fused pitch degrees x10 */
+                    (int)(imu.pitch_rate * 10),  /* gyro pitch rate °/s x10 */
                     (int)bal.motor_cmd,
                     (int)bal.state);
             } else {