Move configuration into package

[OrF8]

Summary

• moves the root-level Constants.py module to statoverflow/config.py
• updates production modules, tests, and notebook imports to use statoverflow.config
• preserves every constant value and repository-relative data/result path
• removes the obsolete root-level configuration module without adding a compatibility shim
• validates all tracked source and notebook code against the Python 3.9 grammar

Python compatibility

• local validation environment: Python 3.12.10
• supported project baseline: Python 3.9
• pyproject.toml remains requires-python = ">=3.9"
• Python 3.10-only annotation syntax was replaced only where required
• no scientific result was regenerated or claimed under Python 3.12

Configuration migration

• statoverflow/config.py is now the canonical configuration module
• BASE_DIR continues to resolve to the repository root
• all derived data, preprocessing, analysis, and cached-output paths are unchanged
• importing configuration performs no filesystem writes
• active code contains no remaining Constants imports

Packaging status

• setuptools package discovery remains disabled
• this PR does not make the source package installable
• editable source installation is deferred to a separate stage

Validation

• created or reused a local Python 3.12 virtual environment
• ran the complete unittest suite
• compared every exported uppercase configuration value before and after
• verified all resolved paths are identical
• verified imports from the repository root and an external working directory
• parsed all tracked Python files with the Python 3.9 grammar
• parsed every notebook code cell with the Python 3.9 grammar
• compiled all Python files with Python 3.12
• validated all five notebooks as JSON
• executed only notebook import/orchestration cells
• confirmed no active Constants imports remain
• confirmed pyproject.toml and requirements.txt are unchanged
• confirmed no data or result files changed
• did not run any real analysis