fix: Windows‑compatible fingerprint verification (closes #17)

[blackdiamond1996]

Summary

Fixes Windows‑specific fingerprint loss by persisting the fingerprint inside the checkpoint dictionary rather than as a model attribute.

Related Issue

Closes #42

Changes

• Added constants and helper functions for injecting/extracting fingerprint data.
• Implemented save_fingerprinted_model and load_fingerprinted_model that are OS‑agnostic.
• Updated verify_fingerprint to read the fingerprint from the checkpoint.
• Added comprehensive unit test test_save_and_verify_fingerprint_cross_platform.

Testing

Ran pytest -q locally – all tests (including the new one) pass.
CI on GitHub Actions also passes.

Checklist

• Code follows repository style (Black/Flake8).
• Added unit tests.
• Updated documentation/comments where needed.