Standing per-component repeal-revenue validation surface (#298)

[MaxGhenis]

Summary

Adopts populace#298's proposal as a standing, diagnostics-only validation surface: every release now computes per-component repeal-revenue deltas and ships them against published benchmarks, so a yield-per-dollar-of-base gap (like QBI's −40% or SALT's −60% on the June dense release) can never again hide behind clean amount residuals.

• repeal_revenue_benchmarks.json — eight components: SALT, home mortgage interest, charitable, medical, casualty, §199A QBI (benchmarked, FY2026), plus tips and overtime with benchmark: null (no published line; the zero-input-base problem stays visible instead of silently untested). Every row is provisional: true with the source string embedding the re-verification requirement (values transcribed from QBI base −31%/−40% vs targets and JCT; add per-component repeal-revenue benchmarks (amount residuals can't see yield gaps) #298's JCX-45-25 table, not yet re-checked against the publication).
• Release runner hook — build_us_fiscal_refresh_release.py::_write_reform_validation() loads the family alongside the existing tax-expenditure and OBBBA reform validations; payload rows carry modeled delta, benchmark, relative gap, and the provisional flag. Strictly non-gating.
• Strict loader + tests — schema/uniqueness/shape validation (neutralized_variable xor parameter_changes), malformed-contract rejections, synthetic arithmetic, null-benchmark handling, diagnostics-only assertion, and a pinned-registry test that every neutralized variable exists in the pinned PolicyEngine-US.

Verification (supervisor, independent)

Full workspace suite with restricted artifacts on this branch: 3,254 passed, 0 failed; ruff clean; resource declared in country_package.json.

Before first certified use

Re-verify the six numeric benchmarks against JCX-45-25 itself (tracked with the in-flight 199A publications research under #530) and decide whether FY2026 lines should be window-adjusted to the release's calendar-year scoring.

Stacked on #534 (base qbi-v2-engine); the PR gate triggers on base=main only, so CI fires as the stack merges down.

Refs #530, closes the standing-surface item of #298

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