fix: restore contracts facade source-size compliance

[Brad-Edwards]

Summary

• extract the contracts facade public-name manifest into a dedicated private module
• reduce contracts/__init__.py from 501 lines to 362 lines without changing its 311 exported names or their order
• add a facade parity test that rejects duplicate or unresolved exports
• fix the source-size policy failure observed in the dev to main promotion PR Dev #919 without adding an allowlist exception

Verification

• canonical nox -s verify graph passed against origin/dev
• 4,992 unit tests passed; 1 skipped
• 55 integration tests passed; 2 environment-dependent tests skipped
• total coverage: 92%
• Sphinx documentation build passed
• staged pre-commit gate passed