feat: add Halt syscall, fault notification to fault handler, supervisor with back-off, shutdown with timeout

- Halt syscall: only pid 0 (init) can halt, adapter uses SBI SRST
- Fault notification: kernel delivers fault info to process fault handler endpoint before killing
- Supervisor: tracks restart counts, back-off up to 5 restarts
- Shutdown: notifies servers, waits with timeout, calls halt
- Trap handler: faults now call kernel.handle_fault instead of halting

crates/kernel/src/syscall/fault.rs

@@ -0,0 +1,66 @@
+use platform::Platform;
+
+use crate::Kernel;
+
+impl<P: Platform> Kernel<P> {
+    pub fn handle_fault(
+        &mut self,
+        tid: types::id::ThreadId,
+        fault: domain::fault::Fault,
+        fault_pc: usize,
+    ) {
+        let pid = match self.threads.get(tid) {
+            Ok(t) => t.process(),
+            Err(_) => return,
+        };
+
+        if let Ok(process) = self.processes.get(pid) {
+            if let Some(handler_slot) = process.fault_handler() {
+                if let Ok(cap) = process.capabilities().get(handler_slot) {
+                    if let domain::resource::Resource::Endpoint(ep_id) = cap.resource() {
+                        let msg = domain::message::Message::from_words([
+                            pid.raw() as usize,
+                            fault.code(),
+                            fault.address(),
+                            fault_pc,
+                        ]);
+
+                        if let Some(ep) = self.endpoints.get_mut(*ep_id) {
+                            if ep.has_waiting_receiver() {
+                                if let Some(receiver_tid) = ep.dequeue_receiver() {
+                                    self.pending_results[receiver_tid.raw() as usize] =
+                                        Some(crate::SyscallResult::Message(msg));
+
+                                    if let Ok(receiver) = self.threads.get_mut(receiver_tid) {
+                                        let priority = receiver.priority();
+                                        let _ = receiver.unblock();
+                                        let core = self.platform.core_id();
+                                        use domain::scheduler::Scheduler;
+                                        self.scheduler.add(receiver_tid, priority, core);
+                                    }
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        let thread_ids: [Option<types::id::ThreadId>; 4] = {
+            match self.processes.get(pid) {
+                Ok(p) => {
+                    let mut ids = [None; 4];
+                    for (i, t) in p.thread_ids().enumerate() {
+                        if i < 4 { ids[i] = Some(t); }
+                    }
+                    ids
+                }
+                Err(_) => return,
+            }
+        };
+
+        self.cleanup_threads(&thread_ids);
+        let _ = self.processes.destroy(pid);
+        self.process_memory[pid.raw() as usize] = None;
+    }
+}