feat: add strict mypy static typing

[henryiii]

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The mypy pre-commit hook ran with no configuration, so it checked nothing. This PR adds a strict [tool.mypy] config to pyproject.toml and annotates the package.

• clang ships no py.typed marker, so a mypy override treats clang.* as untyped and the libclang cursor/TU parameters are Any.
• Strict mode found one latent type error: process_comment sliced strings with leading_spaces = float("inf"); it is now int | None.
• Small refactors for clean types: renamed shadowed variables that held two types (result, indent, library_file), and sections entries are now tuples instead of 2-element lists.
• mypy 2.x does not support python_version = "3.9"; the import was verified on Python 3.9 manually.