fix: kernel hardening pass 2 — page tracking, queue overflow, region tracking

Page lifecycle:
- ProcessMemory now tracks (PhysicalAddress, count) pairs for each
  allocation (up to 16). destroy_process iterates and frees all
  pages back to the pool. No more permanent page leaks on exit/fault.
- RequestPages re-adds MemoryRegion entries so ReleasePages works.
  Release test un-ignored, passes.
- RequestPages: VA allocation moved after pool allocation succeeds,
  preventing VA leak on OOM.

IPC hardening:
- WaitQueue overflow (>16 waiters): returns WouldBlock error instead
  of silently dropping the thread. Thread is not blocked if enqueue
  fails.
- debug_assert on pending_messages[sender].take() — invariant that
  a sender in wait queue must have a pending message.

Scheduler:
- remove() now clears current thread if the removed thread was
  current on its core.
- add() asserts push succeeded; bitmap bit only set if thread was
  actually added to RunList.

Cleanup:
- Priority::DEFAULT replaces Priority::new(16).unwrap() in
  spawn_thread and spawn_thread_in.

36 kernel tests pass, 0 ignored.

crates/domain/src/scheduler/multi_core.rs

@@ -68,7 +68,11 @@ impl Scheduler for MultiCoreScheduler {
     fn remove(&mut self, thread: ThreadId) {
         let idx = thread.raw() as usize;
         if let Some(placement) = self.placements[idx].take() {
-            self.cores[placement.core.raw() as usize].remove(thread, placement.priority);
+            let core_idx = placement.core.raw() as usize;
+            self.cores[core_idx].remove(thread, placement.priority);
+            if self.cores[core_idx].current() == Some(thread) {
+                self.cores[core_idx].clear_current();
+            }
         }
     }