XY-1074: [ELF AMK P4] Add latency, cost, private-corpus, and production-readiness evidence gates

[yvette-carlisle]

Summary

• Add operational evidence gates to real-world job benchmark reports with separate local_fixture, public_proxy, private_corpus, and provider_backed tiers.
• Add P4 production-readiness production_ops fixture/report artifacts and cargo make entrypoint.
• Update benchmark spec/runbook/README/evidence index so private/provider unavailable inputs remain typed blockers, not pass claims.

Validation

• cargo make fmt
• cargo make lint-fix
• cargo make checks
• cargo test -p elf-eval --test real_world_job_benchmark -- --nocapture
• cargo make real-world-memory-p4-production-readiness

Review

• Independent fresh-context read-only Decodex Review recorded clean for HEAD de58910.