ME-RV-081 — Held-out external benchmark

## Summary
External-project held-out set with permissions + provenance.

**Normative:** `10_FOUNDRY_BENCHMARKS_AND_FEDERATION.md` (ME-RV-081)

## Scaffolding
- Issue template: `.github/ISSUE_TEMPLATE/held_out_benchmark.yml`

## Acceptance
- [ ] ≥5 per family with permissions recorded
- [ ] Not copied examples claimed as adoption