M1: the AC5 scanner parses the (field, code, message) literal tuple in
validation.py structurally instead of naming the two known codes — a
third tuple entry with an unregistered code now fails the registry test
(verified with an injected invalid_ghost_entity_mutant_probe), and a
tuple whose string count is not a multiple of three refuses instead of
guessing.
M2: the backend reuse key now includes its direct job inputs introduced
by this issue — scripts/backend-coverage-baseline.txt (the threshold the
comparison step reads), requirements_test.txt (the pip source) and
pyproject.toml (ruff/mypy config) — verified: the key changes when the
baseline changes and is restored byte-for-byte with the file.
User-Visible: no
Issue: #42