Developer and operator tools

These files are not required by the running RustProxy proxy process.

dev/ - local mock services and diagnostic traffic/compression helpers.
audit/ - explicit operator audit/probe tools.
legacy/ - retained compatibility/release-test helpers; not the supported production workflow.

The removed checks/ tree is not shipped. Use ./dev.sh check and ./dev.sh test for Rust validation; source/upgrade preflight lives in scripts/preflight.py.