Test/core crypto primitives

[The-FOOL-00]

Summary

This PR expands unit test coverage across gitlawb-core crypto primitives and DID utilities. It focuses on validating failure paths and invariants around certificate structure, content addressing, DID parsing/validation, and identity signing.

Certificates (cert)

• Adds validation tests for wrong cert type, invalid to hash, and empty signatures
• Verifies tampered signatures fail verify_all
• Confirms threshold behavior for non-maintainer signers and threshold=0

Content IDs (cid)

• Ensures from_sha256_bytes matches from_git_object_bytes
• Adds CID parsing success/failure tests
• Validates distinct inputs produce distinct CIDs
• Adds SHA-256 hex conversion error cases and known empty-hash constant

DIDs (did)

• Confirms did:web and did:gitlawb cannot resolve to verifying keys locally
• Adds parsing validation for missing DID prefix and unsupported methods
• Validates supported method set and predicate helpers
• Verifies method_id extraction

Identity (identity)

• Adds seed round-trip coverage
• Verifies sign_b64 outputs valid base64url signatures
• Ensures wrong-key verification fails
• Adds signed payload round-trip and failure cases
• Adds invalid PEM error-path test

Testing

• cargo test --workspace --exclude gl
• cargo test -p gitlawb-core --doc

Notes

• Full workspace cargo test failed only for the gl test binary due to Windows Application Control (os error 4551). All other tests passed.