Ban inventory: rewrite stub_server.py (runtime-generated test stub) in bash/Rust #5864

Operator, verbatim, 13:20Z (relayed by aprender-57): "5. Ban inventory: stub_server.py, crux_fake_serve.py, fat_driver.py → one ticket each, rewrite in bash/Rust."

stub_server.py is not tracked; it is written at run time and named by scripts/check_crux_ollama_in_lock.sh and scripts/check_harness_server_cleanup.sh (the latter reaps it, and its 'apr stub' row flaked on #5854, x86-main 12:43Z). Rewrite as bash/Rust, drop the Python, update both guards.
Owner: not assigned (wsweep was the owner of the apr-stub flake).