CI: upload canonical artifacts for validation, image parity, and performance

[iahncajigas]

Summary

• add always-run artifact uploads with if-no-files-found: error for validation-pdf, image-mode-parity, and performance-parity workflows
• add workflow-side artifact generation so canonical files exist under output/pdf and output/performance
• add regression guard script tools/reports/assert_artifacts_exist.py and run it before uploads in each workflow

Artifact coverage

• validation-pdf: output/pdf/*.pdf, output/pdf/*.json, output/pdf/*.csv
• image-mode-parity: output/pdf/image_mode_parity/** and pairing JSON under output/pdf/*.json
• performance-parity: output/performance/performance_parity_report.json, output/performance/performance_parity_report.csv, and output/performance/*.json|*.csv

Local checks

• pytest -q
• python tools/reports/assert_artifacts_exist.py --kind validation
• python tools/reports/assert_artifacts_exist.py --kind image
• python tools/reports/assert_artifacts_exist.py --kind performance