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/multi_core.rs

@@ -30,6 +30,13 @@ impl MultiCoreScheduler {
         }
     }
 
+    /// Whether `thread` waits in some core's run queue.
+    pub fn is_queued(&self, thread: ThreadId) -> bool {
+        self.cores[..self.num_cores as usize]
+            .iter()
+            .any(|c| c.is_queued(thread))
+    }
+
     pub fn current(&self, core: CoreId) -> Option<ThreadId> {
         self.cores[core.raw() as usize].current()
     }