StaggeredTripleDifference PR-A: Ortiz-Villavicencio & Sant'Anna (2025) paper review

Add docs/methodology/papers/ortiz-villavicencio-santanna-2025-review.md — the
canonical scholarly review of arXiv:2505.09942v3 ("Better Understanding Triple
Differences Estimators"), the shared primary source for the TripleDifference
(Complete) and StaggeredTripleDifference (In Progress) estimators.

Full-paper, equal-depth review version-pinned to arXiv v3: framework + four
identifying assumptions (S/SO/NA/DDD-CPT), the "three-not-two-DiDs" covariate
result (§3.1) and the not-yet-treated-pooling bias (§3.2), nonparametric
identification (Theorem 4.1 / Eq. 4.5), RA/IPW/DR estimands (Eqs. 4.1-4.4),
the optimal-GMM combination across comparison cohorts (Eqs. 4.11-4.12,
Remark 4.6 RIF), event-study/overall aggregation (Eqs. 4.13-4.14), influence-
function inference (Theorem 4.2, Corollary 4.2), and the three empirical
illustrations. All REGISTRY equation anchors verified accurate against v3.

Also pins the TripleDifference and StaggeredTripleDifference REGISTRY
Primary-source citations to arXiv v3 and adds a one-line "Paper review on
file:" cross-reference (matching the Goodman-Bacon / Roth pattern) so the
review is not orphaned.

Paper-review wave 1 of the 2-PR methodology promotion; no source, test,
CHANGELOG, or tracker-promotion changes — those land in PR-B.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: igerber

docs/methodology/REGISTRY.md

@@ -1951,7 +1951,7 @@ Convergence criterion: stop when objective decrease < min_decrease² (default mi
 
 ## TripleDifference
 
-**Primary source:** [Ortiz-Villavicencio, M., & Sant'Anna, P.H.C. (2025). Better Understanding Triple Differences Estimators. arXiv:2505.09942.](https://arxiv.org/abs/2505.09942)
+**Primary source:** [Ortiz-Villavicencio, M., & Sant'Anna, P.H.C. (2025). Better Understanding Triple Differences Estimators. arXiv:2505.09942v3.](https://arxiv.org/abs/2505.09942v3). Paper review on file: `docs/methodology/papers/ortiz-villavicencio-santanna-2025-review.md`.
 
 **Key implementation requirements:**
 
@@ -2061,7 +2061,7 @@ contract changes.
 
 ## StaggeredTripleDifference
 
-**Primary source:** [Ortiz-Villavicencio, M., & Sant'Anna, P.H.C. (2025). Better Understanding Triple Differences Estimators. arXiv:2505.09942.](https://arxiv.org/abs/2505.09942)
+**Primary source:** [Ortiz-Villavicencio, M., & Sant'Anna, P.H.C. (2025). Better Understanding Triple Differences Estimators. arXiv:2505.09942v3.](https://arxiv.org/abs/2505.09942v3). Paper review on file: `docs/methodology/papers/ortiz-villavicencio-santanna-2025-review.md`.
 
 **Key implementation requirements:**