feat: Add signals, exec/wait, and FD/pipe support

Introduce process lifecycle and POSIX-like features: add parent_pid, pgid, exited/exit_status, signal state and handlers, waitpid/reap, and an exec-replace function. Refactor file descriptor handling to use fd_kind/fd_flags with reference-counted file refs and in-process pipes; implement open/read/write/close/seek/tell/size/dup/dup2/pipe/fcntl semantics and O_* flags. Add syscall handlers for exec, waitpid, kill/signal, sigaction, sigprocmask, sigpending, meminfo/ticks and map many SYSTEM_CMD_* constants; deliver signals from the syscall path. Cleanup/terminate logic updated to free resources correctly and initialize kernel/user processes with new state. Misc: minor syscall/table renames (wallpaper), helper utilities (process_close_fd_inner, process_init_signal_state) and paging/stack handling for exec.

src/userland/sys/reboot.c

@@ -7,6 +7,6 @@
 int main(int argc, char **argv) {
     (void)argc; (void)argv;
     printf("Rebooting...\n");
-    sys_system(12, 0, 0, 0, 0); // SYSTEM_CMD_REBOOT
+    sys_system(SYSTEM_CMD_REBOOT, 0, 0, 0, 0);
     return 0;
 }