refactor: migrate all filesystems to struct file_operations

refactor: migrate all filesystems to struct file_operations

Every filesystem and device driver now defines static const
file_operations tables and assigns f_ops on every node:
- tmpfs: tmpfs_file_ops, tmpfs_dir_ops
- devfs: devfs_dir_ops, dev_null_ops, dev_zero_ops, dev_random_ops
- ext2: ext2_file_fops, ext2_dir_fops
- fat: fat_file_fops, fat_dir_fops
- diskfs: diskfs_file_fops, diskfs_dir_fops
- overlayfs: overlay_file_ops, overlay_dir_ops
- tty: tty_fops (console + tty)
- pipe: pipe_read_fops, pipe_write_fops
- socket: sock_fops
- vbe: fb0_fops
- keyboard: kbd_fops

VFS dispatch (fs.c + syscall.c) checks f_ops first, falls back to
legacy per-node pointers. Legacy pointers are still set (dual
assignment) for callers that access them directly (e.g. overlayfs
layer delegation). Phase B3 will remove legacy pointers after all
direct accesses are eliminated.

20/20 smoke tests pass, cppcheck clean.