Add bundled type stubs

[heejaechang]

Summary

• bundle PEP 561 stubs for the package and its shipped submodules
• type the complete expression API, operators, parsing/token streams, substitutions, evaluation, instance fields, and custom algebra class injection
• add py.typed; the existing package-data configuration includes every typing artifact in wheels and sdists

Validation

• pyright --verifytypes boolean --outputjson: 100% (239 known exports, 0 ambiguous/unknown; baseline was 0% with no py.typed)
• python -m mypy.stubtest boolean: clean across all 3 shipped modules
• positive Pyright/mypy consumer fixture: clean; negative fixture: all 5 invalid contracts rejected by both
• tox -e py314: 86 passed, 4 expected failures
• Black and isort checks: clean
• Sphinx build with warnings as errors: clean
• wheel and sdist build plus twine check: clean; both archives contain __init__.pyi, boolean.pyi, test_boolean.pyi, and py.typed
• packaged runtime .py files are byte-identical to the source checkout