Fix published metadata normalization

[stefan-jansen]

Summary

• accept canonical PEP 621 contacts in both split and RFC 822 PyPI forms
• compare Requires-Python values as semantic specifier sets
• extend the approved Polars exception through the current Data and Engineer releases
• add regression coverage for accepted and rejected identity forms and reordered specifiers

Verification

• uv run ruff check .
• uv run ruff format --check .
• uv run ty check
• uv run pytest --cov --cov-report=term-missing -q
• uv run mkdocs build --strict
• pre-commit run --all-files
• fresh ecosystem collection: Engineer passes all checks at 0.1.4

Closes #15

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