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/userspace/src/syscall.rs

@@ -47,6 +47,11 @@ pub fn revoke(slot: usize) -> usize {
     arch::syscall1(SYS_REVOKE, slot)
 }
 
+pub fn halt() -> ! {
+    arch::syscall0(SYS_HALT);
+    loop {}
+}
+
 pub fn send(endpoint: usize, w0: usize, w1: usize, w2: usize, w3: usize) -> usize {
     arch::syscall5(SYS_SEND, endpoint, w0, w1, w2, w3)
 }