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/types/src/syscall_abi.rs

@@ -15,3 +15,4 @@ pub const SYS_GET_TIME: usize = 13;
 pub const SYS_GET_PROCESS_ID: usize = 14;
 pub const SYS_YIELD: usize = 15;
 pub const SYS_EXIT: usize = 16;
+pub const SYS_HALT: usize = 17;