[Threading] Add thread priority mapping with timer-driven quantum decay

And default ignore_thread_priorities to false.

Map Xenon's 0-31 priority range across all 5 host priority levels
instead of collapsing 0-17 into kNormal.

Use timer-driven quantum decay (~20ms period) matching Xenon's
60-quantum / 3-per-tick cycle to prevent starvation by gradually lowering
effective priority for non-real-time threads (< 18), piggybacking on the
existing 1ms timestamp timer.

src/xenia/base/cvar.h

@@ -511,7 +511,7 @@ class IConfigVarUpdate {
   // If you're reviewing a pull request with a change here, check if 1) has been
   // done by the submitter before merging.
   static constexpr uint32_t kLastCommittedUpdateDate =
-      MakeConfigVarUpdateDate(2025, 12, 4, 21);
+      MakeConfigVarUpdateDate(2026, 4, 9, 12);
 
   virtual ~IConfigVarUpdate() = default;