Enable editable package installation

[OrF8]

Summary

• explicitly packages statoverflow and statoverflow.analysis
• enables editable installation from the repository checkout
• allows project imports from arbitrary working directories without PYTHONPATH
• preserves Python 3.9 as the supported project baseline
• preserves all repository-relative data and result paths

Supported installation mode

• development command: pip install -e ".[dev]"
• editable imports resolve to source files in the checkout
• statoverflow.config.BASE_DIR continues to resolve to the checkout root
• package declarations are explicit rather than automatically discovered

Packaging boundary

• included: statoverflow, statoverflow.analysis
• excluded: tests, notebooks, data, results, and root-level modules
• package data remains disabled
• no console scripts or entry points are added

Limitation

• this PR validates editable checkout installation only
• ordinary wheel installation is not claimed as suitable for repository analysis workflows
• installed-wheel path handling is deferred to a separate design decision

Validation

• used a Python 3.12.10 local environment while preserving Python 3.9 project compatibility
• ran the complete unittest suite
• validated explicit setuptools package declarations
• installed the checkout editably in the project .venv
• validated the dev extra without upgrading dependencies
• verified editable distribution metadata and direct_url.json
• imported all production modules from an external working directory without PYTHONPATH
• confirmed imported source files resolve to the checkout
• confirmed BASE_DIR, DATA_DIR, and RESULTS_DIR are unchanged
• repeated an editable-install smoke test in a fresh temporary virtual environment
• parsed all tracked source and notebook code with the Python 3.9 grammar
• compiled source and tests with Python 3.12
• validated all five notebooks without executing analysis cells
• confirmed requirements.txt, notebooks, data, and results are unchanged
• removed all generated packaging artifacts
• did not build or install a normal wheel
• did not run any real analysis