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/cli/date.c

@@ -7,7 +7,7 @@
 int main(int argc, char **argv) {
     (void)argc; (void)argv;
     int dt[6];
-    if (sys_system(11, (uint64_t)dt, 0, 0, 0) == 0) {
+    if (sys_system(SYSTEM_CMD_RTC_GET, (uint64_t)dt, 0, 0, 0) == 0) {
         printf("Current Date: %d-%d-%d %d:%d:%d\n", dt[0], dt[1], dt[2], dt[3], dt[4], dt[5]);
     } else {
         printf("Error: Could not retrieve date.\n");