refactor: remove namespace callbacks from struct file_operations

refactor: remove namespace callbacks from struct file_operations

Complete the file_operations / inode_operations separation:

- fs.h: struct file_operations now contains only per-fd I/O ops:
  read, write, open, close, ioctl, mmap, poll
- fs.h: struct inode_operations exclusively owns namespace/metadata:
  lookup, readdir, create, mkdir, unlink, rmdir, rename, truncate, link
- Migrated ext2 and initrd (missed in previous commits)
- Removed all f_ops fallback paths in fs.c, syscall.c, overlayfs.c,
  kconsole.c — everything now uses i_ops for namespace operations
- Clean separation: file_operations = fd I/O, inode_operations = namespace

All FSes migrated: diskfs, devfs, procfs, tmpfs, overlayfs, persistfs,
pty, fat, ext2, initrd.

cppcheck clean, 20/20 smoke tests pass.