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/init/src/shutdown.rs

@@ -1,3 +1,25 @@
-pub fn shutdown() -> ! {
-    userspace::syscall::exit();
+const SHUTDOWN_TIMEOUT_TICKS: usize = 50_000_000;
+
+pub fn shutdown(server_endpoints: &[usize], endpoint_count: usize) -> ! {
+    for i in 0..endpoint_count {
+        let ep = server_endpoints[i];
+        if ep != 0 {
+            userspace::syscall::send(ep, 0xFFFF_FFFF, 0, 0, 0);
+        }
+    }
+
+    let start = userspace::syscall::get_time();
+    let deadline = start + SHUTDOWN_TIMEOUT_TICKS;
+
+    let mut acks = 0usize;
+    while acks < endpoint_count {
+        let now = userspace::syscall::get_time();
+        if now >= deadline {
+            break;
+        }
+        acks += 1;
+        userspace::syscall::yield_now();
+    }
+
+    userspace::syscall::halt();
 }