Ban inventory: rewrite scripts/lib/crux_fake_serve.py in bash/Rust #5865

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."

Tracked at scripts/lib/crux_fake_serve.py. Referenced by scripts/check_crux_serve_code.sh and scripts/check_harness_server_cleanup.sh (plus evidence/pr-review receipts, historical, not to be edited). Rewrite as bash/Rust, drop the Python.