feat(kernel): seL4-style IPC, notifications, status ABI

- badges + Mint, Call/Reply/ReplyRecv w/ gen-checked reply slot
- cap transfer (1/msg, copy, needs Delegate, rechecked at delivery)
- notifications: signal/wait/poll/bind, irq -> notification routing
- WaitAny/WaitForInterrupt removed; a0=status, ErrorCode
- libkos: single ecall primitive, compat wrappers + typed new api
- fix: trap return clobbered user t0 on every trap
- fix: wait queues FIFO, delegate keeps badge, fault cap checked
- docs/ipc.md spec, ipc tests, ipc fuzz, kani ntfn + waitqueue

crates/domain/src/scheduler/per_core.rs

@@ -29,6 +29,10 @@ impl PerCoreScheduler {
         self.queue.add(thread, priority);
     }
 
+    pub fn is_queued(&self, thread: ThreadId) -> bool {
+        self.queue.contains(thread)
+    }
+
     pub fn remove(&mut self, thread: ThreadId) {
         self.queue.remove(thread);
     }