Define execution provenance v1 (#69)

[andrewperis]

Summary

• Define the metadata-only Execution Provenance v1 candidate contract, RFC and classification-aware redaction policy.
• Add synthetic valid/invalid fixtures and the constitutional conformance assessment.
• Keep immutable contract publication gated on Studio issue #72.

Verification

• Coordinated Platform suite: 74 tests pass.
• Valid/invalid contract fixtures checked; generated validator is reproducible.

Coordination

Part of DefinitelySecureStudio/studio#69. Merge this contract PR before DefinitelySecureStudio/platform#12. Close the tracker only after both coordinated PRs merge.

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-11T12:32:00.238524Z	6b6f192	Draft marked ready

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Reviewed commit: 6b6f192c4e

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