docs: add CiC/QDiD paper reviews (Athey-Imbens 2006, Callaway-Li-Oka 2018, Melly-Santangelo 2015, Ciaccio 2024)

ROADMAP.md

@@ -32,7 +32,7 @@ Research-informed candidates. Each has a rationale, a tractability note, and a c
 ### Methodology extensions
 
 - **Nonparametric / flexible outcome regression for `EfficientDiD` DR covariate path** (Chen, Sant'Anna & Xie, arXiv:2506.17729, 2025, Section 4). The shipped staggered `EfficientDiD` uses a linear OLS outcome regression in its doubly-robust covariate path; that preserves DR consistency but does not generically attain the semiparametric efficiency bound unless the conditional mean is linear in the covariates. Replacing the OLS outcome regression with sieve / kernel / ML nuisance estimation (as the paper's Section 4 allows) would close the efficiency gap on the covariate path. Tractability: medium; the hook points are in `diff_diff/efficient_did_covariates.py`. **Commit when**: a paper-review synthesis is written, with an implementation plan for the nonparametric OR that preserves the existing DR consistency guarantees and survey-weighted variance surface.
-- **Distributional DiD for staggered timing** (Ciaccio, arXiv:2408.01208, 2024). New estimator extending Callaway-Li QTT to staggered adoption. `CallawaySantAnna` currently gives mean ATT only; this unlocks quantile effects. Tractability: medium. **Commit when**: a health-econ or public-health user reports need for quantile effects in a repeated-cross-section design.
+- **Distributional DiD for staggered timing** (Ciaccio, arXiv:2408.01208, 2024). New estimator extending Callaway-Li QTT to staggered adoption. `CallawaySantAnna` currently gives mean ATT only; this unlocks quantile effects. Tractability: medium. **Reviewed 2026-07** (`docs/methodology/papers/ciaccio-2024-review.md`); implementation deferred pending demand. **Commit when**: a health-econ or public-health user reports need for quantile effects in a repeated-cross-section design.
 - **Local Projections DiD** (Dube, Girardi, Jordà & Taylor, JAE 2025). New estimator with flexible impulse-response and robustness to dynamic misspecification; natural for anticipation-prone settings. Tractability: well-scoped. **Commit when**: a methodology review confirms the dynamic variant's variance derivation fits our SE helpers.
 - **Few-treated-units inference option** (Alvarez, Ferman & Wüthrich, arXiv:2504.19841, 2025). `inference=` option covering t(G-1) corrections, randomization inference, and Ferman-Pinto-style permutation tests. Current SE paths assume large-G asymptotics. Tractability: medium. **Commit when**: a user reports sparse-treatment pain.
 - **Riesz-representation sensitivity** (Bach et al., arXiv:2510.09064, 2025). Confounder-based sensitivity bound complementing HonestDiD's trend-based bound. Tractability: medium. **Commit when**: HonestDiD users ask for confounder bounds.