feat: add validation problem demo cases

[hyeokjun32]

Summary

• Added validation problem-case fixtures for annotation-missing, invalid detection structure, and contract shape mismatch paths.
• Connected problem-case summaries to Local Studio Load Demo Evidence.
• Documented review/block evidence behavior in portfolio docs.

Validation

• poetry run python3 -m pytest tests/test_studio_routes.py tests/test_validation_demo_report.py -q
• poetry run python3 -m pytest -q

Impact

• Shows that InferEdge records review/block evidence instead of only happy-path validation.
• Makes the Local Studio demo more useful for interview walkthroughs by showing normal and problem evidence states.
• Keeps the project local-first without DB, queue, upload, auth, billing, or production SaaS additions.

Notes

• Problem cases are compact JSON report fixtures, not raw dataset files.
• AIGuard remains optional; Lab remains the final decision owner.