fix: kernel concurrency, interrupt context, and user VA safety

kernel/src/Timekeeping/ApicTimer.hpp

@@ -28,6 +28,9 @@ namespace Timekeeping {
     // Enable scheduler tick (called after scheduler is initialized)
     void EnableSchedulerTick();
 
+    // Run bounded process-context bottom halves from an idle CPU.
+    void ServiceDeferredWork();
+
     // Enter one idle wait cycle for the current CPU. Uses HLT and relies on
     // the boot-time periodic LAPIC timer to wake -- one-shot wake doesn't
     // survive deep C-states on some laptop CPUs.