FreeRTOS backend modules

Pigweed provides native FreeRTOS backends for several of its core OS abstraction layers.

OS backend modules

API group	Backend module	Description
Time	pw_chrono_freertos	Non-overflowing 64-bit SystemClock and race-free SystemTimer.
Interrupts	pw_interrupt_freertos	Interrupt service routine utilities for task-to-ISR synchronization.
Memory Allocation	pw_malloc_freertos	Interface wrapper mapping Pigweed allocation to FreeRTOS memory heaps.
Synchronization	pw_sync_freertos	Type-safe Mutex, InterruptSpinLock, Semaphores, and ThreadNotification.
Threading	pw_thread_freertos	Thread creation, stack checking, sleep, yield, and iteration utilities.