data: build public-safe 100-example downstream reranking set

[AbdelStark]

Parent tracker: #209

Goal

Replace the one-example downstream fixture with a public-safe benchmark pack containing at least 100 labeled candidate-reranking examples.

Scope

• Define source selection and license policy for benchmark tasks and candidate patches.
• Preserve fixture mode for tests and local development.
• Emit source/license, split-leakage, readiness, manifest, and secret-scan reports.
• Keep candidate code as untrusted text; do not import or execute it.

Acceptance Criteria

• codelewm eval downstream-pack builds the scaled pack from a checked-in config.
• Readiness reports example_count >= 100 and no source/license or split-leakage blockers.
• Artifacts are JSON-native, checksum-verifiable, and secret-scanned.
• Docs state exactly what labels mean and which claims are still blocked before evaluation.