feat(platforms): Add STM32 platform support (STM32G474RE + STM32F413ZH)

[nhuvaoanh123]

Summary

Proposing to add STM32 as a third platform alongside POSIX and S32K1xx, targeting two Nucleo boards:

Board	MCU	CAN	Clock	Flash/RAM
NUCLEO-G474RE	Cortex-M4F	FDCAN	170 MHz	512K / 128K
NUCLEO-F413ZH	Cortex-M4	bxCAN	96 MHz	1.5M / 320K

BSP Modules Included

• bspCan — register-level FDCAN (G4) + bxCAN (F4) drivers, no HAL dependency
• bspClock — PLL configuration for both chips (HSI/HSE → target freq)
• bspUart — polling UART TX/RX with dual register set support (F4 SR/DR vs G4 ISR/TDR)
• bspTimer — DWT CYCCNT-based system timer (architecture-level, chip-agnostic)
• bspInterruptsImpl — CPSID/CPSIE global interrupt masking
• bspMcu — CMSIS headers, linker scripts, startup assembly, NVIC macros
• bxCanTransceiver / fdCanTransceiver — OpenBSW AbstractCANTransceiver wrappers
• hardFaultHandler — Cortex-M4 register dump to NO_INIT RAM
• softwareSystemReset — NVIC_SystemReset() with pre-reset hook
• RTOS ports: FreeRTOS CM4F SysTick + ThreadX Cortex-M4/GNU

What's Not Yet Included (future PRs)

• bspIo (GPIO abstraction)
• safeBspMcuWatchdog (IWDG driver)
• CMakePresets entries for both boards
• Reference application executables

Validation

• 46/46 unit tests pass (22 BxCanTransceiver + 23 FdCanTransceiver + 1 UART)
• Validated against upstream OpenBSW with directory junctions on MinGW GCC 15.2.0
• CAN loopback self-test verified on NUCLEO-G474RE hardware

Proposed PR Strategy

1. PR 1: Core platform (platforms/stm32/) + F413ZH reference app
2. PR 2: G474RE reference app + FDCAN integration
3. PR 3: GPIO + watchdog modules

Happy to adjust the approach based on maintainer feedback.