XY-818: [ELF prod P0] Add private-corpus production adoption benchmark

[yvette-carlisle]

Summary

• add a production corpus manifest schema and checked-in synthetic coding-agent fixture
• add synthetic/private production corpus cargo-make benchmark entrypoints with fail-closed private manifest handling
• extend ELF benchmark reports with evidence IDs, wrong-result counts, latency, provider, and embedding dimensions

Verification

• cargo make fmt
• cargo make lint-fix
• cargo make checks
• cargo make baseline-production-synthetic
• cargo make baseline-production-private (fails closed without ELF_BASELINE_PRODUCTION_CORPUS_MANIFEST)