refactor: VFS file_operations dispatch layer

refactor: VFS file_operations dispatch layer

Add struct file_operations to fs.h with all VFS callback signatures.
Add const struct file_operations* f_ops to fs_node_t.

Update all VFS dispatch points (fs.c wrappers + syscall.c direct
dispatch for poll, readdir, ioctl, mmap) to check f_ops first,
then fall back to legacy per-node function pointers.

This enables incremental migration: filesystems can adopt f_ops
one at a time while legacy pointers continue to work.

20/20 smoke tests pass.