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/sys/stat.h

@@ -1,11 +1,45 @@
 #ifndef BOREDOS_LIBC_SYS_STAT_H
 #define BOREDOS_LIBC_SYS_STAT_H
 
+#include "types.h"
+
+typedef long time_t;
+
 struct stat {
+    unsigned long st_dev;
+    unsigned long st_ino;
+    mode_t st_mode;
+    unsigned long st_nlink;
+    uid_t st_uid;
+    gid_t st_gid;
+    unsigned long st_rdev;
     int st_size;
-    int st_mode;
+    long st_blksize;
+    long st_blocks;
+    time_t st_atime;
+    time_t st_mtime;
+    time_t st_ctime;
 };
 
+#define S_IFMT   0170000
+#define S_IFIFO  0010000
+#define S_IFCHR  0020000
+#define S_IFDIR  0040000
+#define S_IFREG  0100000
+
+#define S_IRUSR  0400
+#define S_IWUSR  0200
+#define S_IXUSR  0100
+#define S_IRGRP  0040
+#define S_IWGRP  0020
+#define S_IXGRP  0010
+#define S_IROTH  0004
+#define S_IWOTH  0002
+#define S_IXOTH  0001
+
+#define S_ISDIR(m) (((m) & S_IFMT) == S_IFDIR)
+#define S_ISREG(m) (((m) & S_IFMT) == S_IFREG)
+
 int stat(const char *pathname, struct stat *statbuf);
 int fstat(int fd, struct stat *statbuf);
 int mkdir(const char *pathname, int mode);