r6 Medium: AC4 was measurable only on a developer's machine — no
workflow invoked mypy, so a typing regression in any of the six
allowlist modules reached dev unnoticed while the issue claimed
measurable backend quality. Coverage and lint had continuous gates;
typing had a text comparison of a committed list.
The backend job now runs mypy right after ruff, from the same pinned
dependency file (mypy==2.3.1 — an unpinned checker would redden on code
that never changed). The step derives its module list from the
pyproject.toml strict allowlist instead of duplicating it, because a
drifted duplicate is a green step checking the wrong modules, and it
refuses an empty list rather than passing silently.
Guarded twice: a contract test pins all three facts (pinned checker,
a step that really invokes it, list read from pyproject) and the new
typing-gate-stops-running mutant reddens when the invocation is
neutered.
User-Visible: no
Issue: #42
Codex
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