refactor: migrate procfs, persistfs, pty to f_ops (dual-assignment)

refactor: migrate procfs, persistfs, pty to f_ops (dual-assignment)

- procfs: 9 static file_operations tables for root, self, self/status,
  uptime, meminfo, cmdline, pid dirs, pid/status, pid/maps
- persistfs: 2 tables for root dir and counter file
- pty: 4 tables for master, slave, ptmx, pts directory

All fs_node_t nodes in the codebase now have f_ops assigned.
Legacy per-node function pointers retained for backward compat.

20/20 smoke tests pass.