scripts/check_test_wiring.py and scripts/check_security_coverage.py import
tomllib, which landed in Python 3.11. macOS ships /usr/bin/python3 at 3.9, so
`make pre-commit` failed on a clean machine with `ModuleNotFoundError: No
module named 'tomllib'` in test-wiring-check, even though the checkers
themselves are fine.
Add scripts/python_bin.sh, which resolves an interpreter (explicit
RUSTFS_PYTHON, then python3.14..3.11/python3/python on PATH, then a
`uv run --python 3.12 --no-project` fallback) and execs it, failing with the
concrete remediation when nothing usable exists. Route the Make call sites
through RUSTFS_PYTHON_BIN. CI workflows keep calling python3 directly because
their runners already provide 3.11+.
