TODO: Harden diff / bump for library CI

Goal

Evolve model-parser diff and model-parser bump from advisory output into optional CI gates: stricter classification, exit codes, and documented policy for how model-library (and others) should interpret results.

Why

• Libraries want “fail if this would be a MAJOR bump without version bump” or similar policies.
• Edge cases (e.g. floating-point literal normalization, metadata-only churn) need explicit rules.

Scope hints

• Flags sketch: --fail-on major, --json stable schema versioning.
• Extend src/model_parser/semantic_diff.py with tests for each policy branch.
• Cross-link from docs/design/model-library-and-versioning.md.

References

• src/model_parser/cli.py (diff, bump).
• tests/test_semantic_diff.py.

Acceptance sketch

• Documented policy table + tests for CI-oriented flags.
• Example GitHub Actions snippet in model-library or parser deployment docs.