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/README.md

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+# CAN FD CDC Composite Firmware
+
+A Zephyr-based firmware for the EWS board that provides:
+- CAN FD to gs_usb interface (similar to candlelight)
+- USB CDC interface for PFET control
+- USB composite device functionality
+
+## Features
+
+- **CAN FD Support**: Full CAN FD protocol support via gs_usb interface
+- **USB Composite**: Single USB device with multiple interfaces:
+  - gs_usb interface for CAN communication
+  - CDC ACM interface for PFET control and status
+- **PFET Control**: Control both output PFETs via CDC commands
+- **Status LEDs**: Control status LEDs for visual feedback
+- **Compatible**: Works with standard CAN utilities (can-utils, etc.)
+
+## Hardware Target
+
+- **MCU**: STM32G0B1KBU6 (on EWS board)
+- **CAN**: CAN FD via FDCAN1 (PB0/PB1) with SN65HVD230 transceiver
+- **USB**: USB 2.0 Full Speed (PA11/PA12)
+- **GPIOs**: 
+  - PFET1 control: PA8
+  - PFET2 control: PB2  
+  - Status LED: PB4
+
+## Build Requirements
+
+- Zephyr RTOS (v3.5+)
+- West build tool
+- ARM GCC toolchain
+
+## Building
+
+```bash
+west build -b ews_board
+```
+
+## CDC Protocol
+
+The CDC interface uses simple text commands:
+- `PFET1_ON\n` - Turn on PFET1
+- `PFET1_OFF\n` - Turn off PFET1  
+- `PFET2_ON\n` - Turn on PFET2
+- `PFET2_OFF\n` - Turn off PFET2
+- `STATUS\n` - Get current PFET status