Add shared PendSV deferred service dispatcher

[crabel99]

Summary

Adds a shared PendSV deferred-service dispatcher to the core.

What changed

• Adds PendSV.h and PendSV.cpp.
• Provides a small service registry for up to 32 deferred service handlers.
• Allows interrupt-context code to mark a service pending with setPending().
• Dispatches pending services from PendSV_Handler() instead of doing heavier work directly in an ISR.
• Tracks pending counts per service so repeated ISR events are not collapsed into a single callback.
• Limits dispatch work per PendSV entry to avoid starving normal thread-mode execution.

Why

This creates a reusable core primitive for libraries that need ISR-safe deferred work, especially DMA and async peripheral paths.

Validation

• metro_m0: 24 succeeded, 0 failed, 37 skipped
• metro_m4: 20 succeeded, 0 failed, 41 skipped
• git diff --check: clean