Add shared C2PA text normalization helpers

[erik-sv]

Summary

• add a shared text_hashing helper that performs NFC normalization, exclusion filtering, and SHA-256 hashing for C2PA text assets
• refactor the C2PA embedding/verification flows to call the helper and stabilise recorded exclusion spans
• document the new helper across the C2PA guides and add regression tests for the hashing workflow

Testing

• pytest tests/interop/test_text_hashing.py tests/integration/test_c2pa_text_embedding.py