Add fixture integrity validation

[Will-thom]

Summary

• add a focused fixture integrity test for demo and testdata artifacts
• guard intentionally committed target report and log fixtures against missing or unexpected files
• document the focused fixture validation command in testing and fixture docs

Validation

• go test ./pkg/prmaven -run TestFixtureIntegrity -v
• git diff --check
• .github/scripts/check-public-metadata.sh
• go test ./...

Closes #70

[github-actions]

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