Add longterm profile and signed evidence validation

[anth-volk]

Summary

• adds the us-longterm-crfb profile to the 4.10.0 archived bundle
• keeps long-running datasets at hash_pinned because representative H5 files were not available locally for byte/runtime verification
• records the long-term partial-certification check as skipped in the archive validation report, with representative years 2026, 2050, and 2100
• adds validation for signature evidence records
• adds tests proving the profile is present without making a partial-certification claim yet

Stacked on #9.

Validation

• PYTHONPATH=src /opt/miniconda3/bin/python -m pytest
• PYTHONPATH=src /opt/miniconda3/bin/python scripts/validate_models.py
• PYTHONPATH=src /opt/miniconda3/bin/python scripts/validate_schemas.py

Note: pytest passes with two Python 3.14 tarfile deprecation warnings inherited from the release extraction tests.