Add Goodman-Bacon (2021) paper review

Comprehensive review of "Difference-in-differences with variation in
treatment timing" (Journal of Econometrics 225(2), 254-277,
DOI: 10.1016/j.jeconom.2021.03.014) targeting the BaconDecomposition
methodology-review promotion. Covers:

- Theorem 1 decomposition (Eqs. 10a-g) with the three 2x2 comparison
  types: treated/untreated, earlier/later, later/earlier
- Variance terms (Eqs. 7-9) and decomposition weights (Eqs. 10e-g)
  with explicit superscript-convention note on which group plays the
  treatment role
- Timing-only pair simplification (Eq. 11, μ_kℓ)
- Probability-limit decomposition VWATT + VWCT - ΔATT (Eqs. 14a-c, 15)
  with the VWCT approximation (Eq. 19) — w_k^T and w_k^C formulas
  written with paper-faithful superscripts (all `k` for treatment-role
  weights, all `j` for control-role weights)
- Linear trend-break wrong-sign case (Eqs. 17-18)
- Constant-ATT identification (Eq. 16)
- Controlled DD extension (Section 5.2, Eqs. 21-27)
- Oaxaca-Blinder-Kitagawa specification-comparison decomposition
  (Eq. 20)

Includes a "Methodology Registry Entry" block formatted to match the
REGISTRY.md structure, marked as **proposed** — not yet merged into
docs/methodology/REGISTRY.md. The audit-pass PR for diff_diff/bacon.py
(tracked in TODO.md "Tech Debt from Code Reviews" → Methodology/
Correctness) carries the REGISTRY replacement plus six concrete
follow-up items (verify Theorem 1 / Eqs 10a-g, decide on always-
treated handling, R parity fixtures, methodology test file, REGISTRY
replacement, METHODOLOGY_REVIEW.md status flip).

Documents two library-vs-paper deviations explicitly:

1. Unbalanced panels: paper Appendix A assumes balanced panels;
   diff_diff/bacon.py:491-499 accepts unbalanced panels with a
   UserWarning. The decomposition is exact only on balanced panels.

2. Always-treated unit U-bucketing: paper footnote 11 puts always-
   treated units (t_i < 1) into U alongside never-treated; diff_diff/
   bacon.py:437-439 documents only first_treat ∈ {0, np.inf} as the
   U sentinels, and bacon.py:504-507 implements only that mask.
   Genuinely always-treated units (0 < first_treat <= min(time), per
   docs/troubleshooting.rst:739-747) are not automatically remapped.
   The audit needs to decide how to handle this gap.

Adds a TODO.md row under Methodology/Correctness tech debt for the
BaconDecomposition methodology-audit follow-up, listing the six
items the audit needs to address.

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

Author: igerber

TODO.md

@@ -74,6 +74,7 @@ Deferred items from PR reviews that were not addressed before merge.
 
 | Issue | Location | PR | Priority |
 |-------|----------|----|----------|
+| BaconDecomposition: substantive methodology audit pass against Goodman-Bacon (2021). Paper review on file at `docs/methodology/papers/goodman-bacon-2021-review.md` includes a proposed Methodology Registry Entry, but the `REGISTRY.md` `## BaconDecomposition` section (lines ~2598-2654) still carries the older contract. Audit pass needs to (a) verify `bacon.py` matches Theorem 1 / Eqs. 10a-g exactly, (b) decide how to handle genuinely always-treated units (`0 < first_treat <= min(time)`) — paper convention puts them in `U`, but `bacon.py` currently treats only `first_treat ∈ {0, np.inf}` as the `U` bucket, (c) generate R parity fixtures via `bacondecomp::bacon()`, (d) write `tests/test_methodology_bacon.py`, (e) replace the REGISTRY entry with the proposed text, (f) populate Verified Components / Corrections Made / Deviations in `METHODOLOGY_REVIEW.md` and flip status to Complete. | `diff_diff/bacon.py`, `docs/methodology/REGISTRY.md`, `tests/test_methodology_bacon.py`, `METHODOLOGY_REVIEW.md` | follow-up | Medium |
 | dCDH: Phase 1 per-period placebo DID_M^pl has NaN SE (no IF derivation for the per-period aggregation path). Multi-horizon placebos (L_max >= 1) have valid SE. | `chaisemartin_dhaultfoeuille.py` | #294 | Low |
 | dCDH: Survey cell-period allocator's post-period attribution is a library convention, not derived from the observation-level survey linearization. MC coverage is empirically close to nominal on the test DGP; a formal derivation (or a covariance-aware two-cell alternative) is deferred. Documented in REGISTRY.md survey IF expansion Note. | `chaisemartin_dhaultfoeuille.py`, `docs/methodology/REGISTRY.md` | #408 | Medium |
 | dCDH: Parity test SE/CI assertions only cover pure-direction scenarios; mixed-direction SE comparison is structurally apples-to-oranges (cell-count vs obs-count weighting). | `test_chaisemartin_dhaultfoeuille_parity.py` | #294 | Low |