feat: power down dGPUs on hybrid graphics laptops

kernel/src/Timekeeping/ApicTimer.hpp

@@ -43,4 +43,12 @@ namespace Timekeeping {
 
     // Busy-wait sleep for the given number of milliseconds
     void Sleep(uint64_t ms);
+
+    // Spin for at least `ms` milliseconds without descheduling. Usable from
+    // boot code, before the scheduler runs, and with interrupts disabled
+    // (falls back to timed I/O port writes when the tick is not advancing).
+    void BusyWaitMs(uint64_t ms);
+
+    // Spin for at least `us` microseconds. Port-0x80 timed, so it errs long.
+    void BusyWaitUs(uint64_t us);
 };