Add survey real-data validation against R using federal survey datasets

Validate diff-diff's survey variance estimation against R's survey package
using three real-world datasets: California API (strata+FPC), NHANES ACA
young adult coverage (strata+PSU+nest), and RECS 2020 (JK1 replicate
weights). All 15 tests match R to machine precision (<1e-10 differences).

Includes R benchmark scripts, Python download scripts, golden value JSON
files, and a real-data section in the survey tutorial demonstrating the
ACA dependent coverage provision DiD on actual CDC data.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: igerber

.gitignore

@@ -55,6 +55,7 @@ Thumbs.db
 
 # Benchmarks - generated data and results (can be regenerated)
 benchmarks/data/synthetic/*.csv
+benchmarks/data/real/raw/
 benchmarks/results/
 
 # Rust build artifacts

CHANGELOG.md

@@ -7,6 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- **Survey real-data validation** (Phase 9) — 15 cross-validation tests against R's `survey` package using three real federal survey datasets:
+  - **API** (R `survey` package): TSL variance with strata, FPC, subpopulations, covariates, and Fay's BRR replicates
+  - **NHANES** (CDC/NCHS): TSL variance with strata + PSU + nest=TRUE, validating the ACA young adult coverage provision DiD
+  - **RECS 2020** (U.S. EIA): JK1 replicate weight variance with 60 pre-computed replicate columns
+  - All metrics (ATT, SE, df, CI) match R to machine precision (< 1e-10 differences)
+
 ## [2.8.4] - 2026-04-04
 
 ### Added