Add initial CAN FD CDC composite firmware

- Custom board definition for STM32G0B1KBU6 (ews_board)
- Zephyr-based firmware with modular architecture
- USB composite device: gs_usb CAN FD + CDC ACM interfaces
- PFET control via CDC text commands (PA8, PB2)
- Status LED on PB4, CAN FD on PB0/PB1
- No external crystal - uses HSI with USB clock recovery
- Ready for build testing with: west build -b ews_board

firmware/canfd_cdc_composite/prj.conf

@@ -0,0 +1,28 @@
+CONFIG_USB_DEVICE_STACK=y
+CONFIG_USB_DEVICE_COMPOSITE=y
+
+# USB CDC ACM
+CONFIG_USB_CDC_ACM=y
+CONFIG_SERIAL=y
+CONFIG_UART_CONSOLE=n
+CONFIG_USB_UART_CONSOLE=y
+
+# CAN configuration
+CONFIG_CAN=y
+CONFIG_CAN_FD_MODE=y
+
+# Networking for gs_usb
+CONFIG_NETWORKING=y
+CONFIG_NET_SOCKETS=y
+CONFIG_NET_SOCKETS_CAN=y
+
+# GPIO for PFET control
+CONFIG_GPIO=y
+
+# Logging
+CONFIG_LOG=y
+CONFIG_USB_DEVICE_LOG_LEVEL_DBG=y
+
+# System
+CONFIG_MAIN_STACK_SIZE=2048
+CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=2048