Address PR #451 R1 review (1 P2, 1 P3)

P2 (Documentation/Tests): the "PDF reviewed" line referenced
papers/goodman-bacon.pdf, which is under the gitignored /papers/
directory and therefore not reproducible from the tree. Replaced
the local-path citation with the Journal of Econometrics version
referenced by DOI 10.1016/j.jeconom.2021.03.014, with an explicit
note that local PDFs are gitignored. This matches the pattern in
rambachan-roth-2023-review.md and wooldridge-2023-review.md
(DOI/URL citations) rather than the local-path pattern in the
older butts/clarke/conley reviews.

P3 (Maintainability): the "source-of-truth target" / "authoritative
reference for the audit work" framing at L14 created dual contracts
with REGISTRY.md until the audit PR lands. Reworded to:
- "proposed replacement text for a future REGISTRY update"
- explicit statement that REGISTRY.md remains the sole authoritative
  contract until the audit PR lands
- pointer to the TODO.md row tracking that audit

No estimator, weighting, variance, or inference code changed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

docs/methodology/papers/goodman-bacon-2021-review.md

@@ -2,7 +2,7 @@
 
 **Authors:** Andrew Goodman-Bacon
 **Citation:** Goodman-Bacon, A. (2021). Difference-in-differences with variation in treatment timing. *Journal of Econometrics*, 225(2), 254-277. DOI: [10.1016/j.jeconom.2021.03.014](https://doi.org/10.1016/j.jeconom.2021.03.014)
-**PDF reviewed:** papers/goodman-bacon.pdf (24 pages: 14 main-text pages, 6 appendix/figure pages with Section 5.2 + Conclusion + Appendix A proof, 4 reference pages)
+**PDF reviewed:** Journal of Econometrics version (DOI: [10.1016/j.jeconom.2021.03.014](https://doi.org/10.1016/j.jeconom.2021.03.014)) — 24 pages total: 14 main-text pages (Sections 1-5.1), 6 appendix/figure pages (Section 5.2 DD with controls, Section 6 Conclusion, Appendix A proof of Theorem 1), 4 reference pages. Local PDFs are gitignored under `/papers/`; the journal/DOI version is the authoritative source.
 **Review date:** 2026-05-16
 
 ---
@@ -11,7 +11,7 @@
 
 *Formatted to match `docs/methodology/REGISTRY.md` structure. Heading levels and labels align with existing entries.*
 
-**Status: proposed.** This block is the source-of-truth target for the REGISTRY entry; it has **not** been merged into `docs/methodology/REGISTRY.md` yet. The current `## BaconDecomposition` section in `REGISTRY.md` (lines ~2598-2654 as of this review) predates this paper-review pass and should be replaced by the corrected content below as part of the follow-up methodology-audit PR for `diff_diff/bacon.py`. Until that PR lands, REGISTRY readers will see the older contract; this paper review is the authoritative reference for the audit work.
+**Status: proposed replacement text for a future REGISTRY update.** This block has **not** been merged into `docs/methodology/REGISTRY.md` yet. The current `## BaconDecomposition` section in `REGISTRY.md` (lines ~2598-2654 as of this review) remains the **sole authoritative methodology contract** until the follow-up audit PR for `diff_diff/bacon.py` lands and replaces it. That audit PR is tracked under `TODO.md` "Tech Debt from Code Reviews" → Methodology/Correctness; it will carry the REGISTRY replacement alongside the paper-vs-code verification, R parity fixtures, methodology test file, and `METHODOLOGY_REVIEW.md` status flip.
 
 ## BaconDecomposition