STM32Cube ld script provides _estack but not __stack_end. Define
__stack_end = _estack - 0x1000 (_Min_Stack_Size) via --defsym so
fault_mpu_guard_init() and fault_mem_c() can locate the stack bottom.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>