Add OKF summary policy receipt fixtures

[caioribeiroclw-pixel]

Summary

• add tiny OKF summary-policy consumer-receipt fixtures for semantic inversion, entity substitution, and provenance erasure
• each case has concept.md, good-summary.md, bad-summary.md, and expected.yaml
• keep the fixture contract consumer-agnostic: it checks preservation against declared summary_policy, not truth in the world

Context

This follows the discussion in #53 about pairing source-side summary_policy with consumer-side receipts/conformance fixtures.

Validation

• parsed all expected.yaml files with Python/YAML
• checked each concept.md has OKF frontmatter and summary_policy
• git diff --check

Note: I could not run the full OKF pytest suite in this environment because pytest is not installed.

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[caioribeiroclw-pixel]

Closing this synthetic fixture PR as superseded by #208, which preserves the four-file contract and replaces the constructed cases with publicly sourced observed cases. #208 is still unreviewed and its CLA check is not yet green, so this closure avoids competing variants; it does not imply maintainer acceptance or merge readiness.