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/CMakeLists.txt

@@ -0,0 +1,11 @@
+cmake_minimum_required(VERSION 3.20.0)
+
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(ews_canfd_cdc_composite)
+
+target_sources(app PRIVATE
+    src/main.c
+    src/cdc_handler.c
+    src/pfet_control.c
+    src/gs_usb_can.c
+)