Add MIT license and document repository licensing

[OrF8]

Summary

• Adds the standard MIT License for the original StatOverflow source code and software documentation.
• Documents the scope of the license in the README.
• Clarifies that the Stack Overflow/SNAP dataset, derived data, third-party research materials, presentation, and logo are not relicensed under MIT and remain subject to their respective terms.
• Adds modern PEP 639 package metadata using the SPDX identifier MIT.
• Includes the LICENSE file in package metadata through license-files.
• Updates the package metadata tests to validate the declared license and confirm that the license file exists.

Packaging Metadata

The project now declares:

license = "MIT"
license-files = ["LICENSE"]

The build-system requirement is updated to a setuptools version supporting SPDX license expressions and project.license-files.

License Scope

The MIT License applies to the original source code and software documentation authored for this project.

The following materials are not relicensed under MIT:

• the Stack Overflow/SNAP source dataset;
• derived data and externally sourced material;
• third-party research content;
• the project presentation;
• the project logo.

These materials remain subject to their original licenses, attribution requirements, or copyright notices.

Validation

• Editable installation completed successfully.
• pip check reported no broken requirements.
• All 145 tests passed.
• git diff --check passed.
• Package metadata correctly reports the MIT SPDX license expression.
• The LICENSE file is included in the built package metadata.
• Only LICENSE, README.md, pyproject.toml, and tests/test_pyproject.py differ from main.
• No source modules, notebooks, datasets, results, presentation files, logo assets, or Git LFS configuration were changed.