fix(packaging): preserve src wheel import contract

[Pigbibi]

Summary

• Package the public src and src.* import contract from the repository root.
• Add a clean-cwd wheel install/import CI gate for src.backtest and lifecycle orchestration.

Validation

• Clean-wheel dependency-enabled install/import from a checkout-external cwd.
• CI gate script executed locally with a Python shim because this workstation exposes python3 but not python.
• Ruff, compileall, unittest discovery, and pytest.