Add shared contracts and contribution checks

[ma7dev]

Summary

• Pin four versioned shared schemas to the merged benchmark contract commit, with CI drift checks against immutable upstream bytes.
• Add a deterministic original eight-case Arabic support-routing development example, dataset manifest and task pack.
• Document contribution requirements for source provenance, generation, versioning and contract changes.

Verification

• 30 tests pass, including exact regeneration, schema validation, case IDs/digests and baseline.
• Changed Python files pass Ruff; all four upstream schema hashes verified.
• Existing Hugging Face release and historical data remain unchanged.

Scope

This completes the dataset portion of Step 2. The example is original synthetic development data, not a held-out model-quality evaluation.