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/libc/libui.h

@@ -22,6 +22,10 @@
 #define GUI_CMD_WINDOW_SET_TITLE 15
 #define GUI_CMD_SET_FONT         16
 #define GUI_CMD_DRAW_STRING_SCALED_SLOPED 18
+#define GUI_CMD_GET_SCREEN_SIZE  50
+#define GUI_CMD_GET_SCREENBUFFER 51
+#define GUI_CMD_SHOW_NOTIFICATION 52
+#define GUI_CMD_GET_DATETIME     53
 
 // Event Types
 #define GUI_EVENT_NONE        0