Add data integrity tools (checksum manifest builder and comparator), update README, and modernize package

[jordanpadams]

🗒️ Summary

This PR adds a full data integrity verification workflow for S3 bucket migrations and improves the existing CLI tooling:

New tools:

• pdc-build-checksum-manifest — generates a CSV manifest of S3 objects with checksums (prefers CRC64NVME, falls back to others); supports --resume-from for incremental runs
• pdc-compare-manifests — compares two manifests by matching (size, checksum_type, checksum_value) tuples; identifies objects in OLD missing from NEW, handles multipart ETag detection, outputs missing/unverifiable/weak CSVs

Improvements:

• Expanded README with full usage docs for all CLI tools including pdc-s3-download, pdc-build-checksum-manifest, and pdc-compare-manifests
• Improved compare_s3_manifests CLI help strings (explicit output file paths, default values) and added progress status output throughout main()
• Added scripts/csv-converter.py utility script
• Added CLAUDE.md project guidance file
• Replaced deprecated pkg_resources with importlib.resources in __init__.py
• Registered new entry points in setup.cfg

🤖 AI Assistance Disclosure

• No AI assistance used
• AI used for light assistance (e.g., suggestions, refactoring, documentation help, minor edits)
• AI used for moderate content generation (AI generated some code or logic, but the developer authored or heavily revised the majority)
• AI generated substantial portions of this code

Estimated % of code influenced by AI: 80%

⚙️ Test Data and/or Report

Manual verification:

• Ran pdc-build-checksum-manifest --help and against a sample S3 bucket
• Ran pdc-compare-manifests against real migration manifests; confirmed correct identification of missing, unverifiable, and weak-checksum objects
• Ran pdc-compare-manifests --help to confirm updated help text and progress output at each stage

♻️ Related Issues

🤓 Reviewer Checklist

Reviewers: Please verify the following before approving this pull request.

Documentation and PR Content

• Documentation: README, Wiki, or inline documentation (Sphinx, Javadoc, Docstrings) have been updated to reflect these changes.
• Issue Traceability: The PR is linked to a valid GitHub Issue
• PR Title: The PR title is "user-friendly" clearly identifying what is being fixed or the new feature being added, that if you saw it in the Release Notes for a tool, you would be able to get the gist of what was done.

Security & Quality

• SonarCloud: Confirmed no new High or Critical security findings.
• Secrets Detection: Verified that the Secrets Detection scan passed and no sensitive information (keys, tokens, PII) is exposed.
• Code Quality: Code follows organization style guidelines and best practices for the specific language (e.g., PEP 8, Google Java Style).

Testing & Validation

• Test Accuracy: Verified that test data is accurate, representative of real-world PDS4 scenarios, and sufficient for the logic being tested.
• Coverage: Automated tests cover new logic and edge cases.
• Local Verification: (If applicable) Successfully built and ran the changes in a local or staging environment.

Maintenance

• Backward Compatibility: Confirmed that these changes do not break existing downstream dependencies or API contracts (or that breaking changes are clearly documented).