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/tests/mock_platform.rs

@@ -54,4 +54,7 @@ impl Platform for MockPlatform {
         _permissions: PermissionSet,
     ) {
     }
+    fn halt(&self) -> ! {
+        loop {}
+    }
 }