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/mod.rs

@@ -1,4 +1,5 @@
 mod capability;
+mod fault;
 mod ipc;
 mod lifecycle;
 mod memory;
@@ -53,6 +54,7 @@ impl<P: Platform> Kernel<P> {
             Syscall::WaitForInterrupt { irq } => self.handle_wait_for_interrupt(tid, pid, irq),
             Syscall::AcknowledgeInterrupt { irq } => self.handle_ack_interrupt(pid, irq),
             Syscall::WaitAny { sources } => self.handle_wait_any(tid, pid, sources),
+            Syscall::Halt => self.handle_halt(pid),
         }
     }
 }