Refactor: consolidate comparison logic into __init__.py

[JustusRijke]

Summary

• Merged comparators.py into __init__.py to consolidate the main comparison logic
• Removed the compare_pdfs function and integrated its logic directly into the diffpdf function
• Updated CLI to call diffpdf directly instead of through a separate comparators module
• Consolidated test files: moved API tests to test_api.py and CLI-specific tests to test_cli.py
• Deleted obsolete files: comparators.py and test_comparators.py

Test plan

• All 10 tests pass
• Code formatted with ruff
• No breaking changes to public API

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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