Ban inventory: rewrite scripts/ci/fat_driver.py in bash/Rust #5866

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/ci/fat_driver.py. 39 files on origin/main reference it (count via git grep -l fat_driver.py origin/main), including the archive wiring in ci/sections.yml and ci.yml (#5812 uses it as the driver). Rewrite as bash/Rust; plan the cutover so #5812's driver step moves with it.