added output contract tests

[iamb4uc]

Summary

• add reusable output contract checks to bin/test
• validate raw/, normalized/, report.md, findings.json, and summary.txt for generated Linux outputs
• verify findings.json structure and allowed severity values without jq
• compare summary finding counts with findings.json

Validation

• ./bin/test syntax
• ./bin/test help
• ./bin/test linux-fixtures
• shellcheck through ubuntu:24.04 Docker
• linux-feasibility through ubuntu:24.04 Docker