Mutation testing cannot attribute mutants for any scripts/ module (synthetic module names in tests_py/scripts)

[cdeust]

What

scripts/mutation_check.sh cannot mutate anything under scripts/. The run
aborts before producing a result:

$ scripts/mutation_check.sh tests_py/scripts/test_launcher_deps.py scripts/launcher_deps.py
Stopping early, because tests recorded trampoline hits but none match any mutant key.
It looks like tests import the source under a different module path than mutmut sees
from the file path.
Recorded keys (e.g.): ['_cortex_launcher_deps.x__importable', ...]
Expected keys (e.g.): ['scripts.launcher_deps.x__importable', ...]

Cause: every test module under tests_py/scripts/ loads its subject with
importlib.util.spec_from_file_location(...) under a synthetic module name
(_cortex_launcher_deps, and the same pattern in
test_generate_pip_constraints.py), while mutmut keys mutants by the path-derived
name scripts.launcher_deps. The trampoline is therefore never matched.

The path-import is deliberate for launcher_deps.py itself — it is loaded by
scripts/launcher.py before the plugin's dependencies exist on sys.path, so it
must stay importable by path at runtime. Nothing forces the TESTS to load it that
way, but changing them is a test-infrastructure change with its own blast radius,
not a line in a feature PR.

Consequence: §12's per-commit mutation gate is silently inapplicable to the entire
scripts/ tree — which holds the launcher's dependency bootstrap, the pip
constraint generator, the doc-claim gate and the badge generators. mutmut fails
loudly here (good), but the gate is simply absent for those modules.

Found while adding python-dateutil to _BASE_PACKAGES in #252 / PR #259 — the
scoped mutation run for that change could not execute.

Acceptance criteria

1. scripts/mutation_check.sh <test> <scripts/...> produces a real mutant tally
   (killed/survived) for at least scripts/launcher_deps.py and
   scripts/generate_pip_constraints.py.
2. launcher_deps.py remains loadable by path at runtime (the launcher's
   pre-dependency bootstrap must not regress) — assert it, don't assume it.
3. Any repository file a tests_py/scripts/ test reads (uv.lock, the doc-claim
   sources) is listed in [tool.mutmut] also_copy, so mutants fail on the
   mutation and not on FileNotFoundError.
4. Surviving mutants in the touched modules are triaged per §12.4: killed, or
   documented equivalents.