feat: infrastructure — pyproject.toml, requirements-dev, conftest, pytest-cov

Ajoute pyproject.toml (pytest strict markers, coverage fail_under=55),
requirements-dev.txt (pytest-cov, pytest-xdist), et tests/conftest.py
avec fixtures partagées (dossier_minimal, dossier_complet, controle_cpam).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

requirements-dev.txt

@@ -0,0 +1,3 @@
+-r requirements.txt
+pytest-cov>=4.1.0
+pytest-xdist>=3.5.0