docs(todo): waive the TWFE full-dummy dedup row (premise resolved by #655)

TODO.md

@@ -30,7 +30,6 @@ The `Origin` column (Actionable tables) and the `PR` column (Deferred tables) bo
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 | `SyntheticControl` conformal (CWZ 2021) AR / innovation-permutation path (Lemmas 5-7) for time-series proxies — the residual-permutation shortcut is only valid for time-permutation-invariant proxies (SC/Lasso/DiD); an AR proxy needs innovation permutation. One-sided alternatives (Remark 1 signed statistic) and proxy covariates (eq 4/6 note) SHIPPED 2026-07. | `diff_diff/conformal.py`, `diff_diff/synthetic_control_results.py` | CWZ-2021 | Heavy | Low |
 | `ContinuousDiD` CGBS-2024 extensions. (a) `covariates=` kwarg — **DONE (reg/dr)**; (b) discrete-treatment saturated regression (`treatment_type="discrete"`) — **DONE**; (c) lowest-dose-as-control per Remark 3.1 when `P(D=0)=0` (`control_group="lowest_dose"`) — **DONE** (discrete + continuous mass-point, single-cohort; estimand `ATT(d)−ATT(d_L)`; see REGISTRY Note #7). Remaining (all deferred `NotImplementedError`, documented): `estimation_method="ipw"` on the dose curve (scalar-adjustment / degenerate); `covariates=` × `survey_design=` (weighted OR + weighted nuisance IF); multi-cohort **heterogeneous-support** discrete aggregation (support-aware: average each dose only over the cohorts that observe it); **multi-cohort `lowest_dose`** (within-cohort `d_L` reference + support-aware cross-cohort aggregation); and **`covariates=` × `lowest_dose`** (conditional-PT-relative-to-`d_L` estimand). Single-cohort / 2-period / shared-support multi-cohort are supported. | `continuous_did.py` | CGBS-2024 | Heavy | Low |
-| TWFE's HC2/HC2-BM inline full-dummy build (`twfe.py:280-315`) duplicates the dummy-construction logic in `DifferenceInDifferences(fixed_effects=...)` (`estimators.py:478-486`). Extract a shared helper, or delegate TWFE's HC2/HC2-BM path to DiD's `fixed_effects=` branch (with TWFE-specific cluster-default threading), to reduce drift risk on FE naming / survey behavior / result-surface conventions. Substantive refactor — touches both estimators. | `twfe.py::fit`, `estimators.py::DifferenceInDifferences.fit` | follow-up | Heavy | Low |
 
 ### Performance
 
@@ -144,6 +143,7 @@ Doable in principle, but no current caller and/or explicitly out of paper scope.
 | **R-script-per-test consolidation has no CI impact.** No CI workflow installs R, so every R-parity test skips in CI behind a per-file availability gate — consolidating `Rscript` spawns yields zero CI speedup. `test_methodology_twfe.py` already session-caches its R fits. The only residual is a LOCAL-dev micro-opt for `test_methodology_continuous_did.py` / `test_methodology_callaway.py` (re-spawn `library(...)` per call). Low value; retained as a local-dev note. | `tests/test_methodology_continuous_did.py`, `tests/test_methodology_callaway.py` | #139 / 2026-06-07 |
 | **`HeterogeneousAdoptionDiD` mass-point IV bread is non-symmetric — `_rank_guarded_inv` inapplicable.** The structural rank-guard sweep (continuous_did / two_stage / spillover / conley) excluded `had.py`'s `ZtWX = Zd'WX` ([1, instrument]' × [1, endogenous]): it is a non-symmetric 2×2 Wald-IV bread (`V = ZtWX_inv @ Omega @ ZtWX_inv.T`), and `_rank_guarded_inv` assumes a **symmetric PSD** Gram (symmetric `D=diag(A)` equilibration + eigendecomposition), so applying it would be methodologically wrong. The existing fallback already returns NaN SE on a singular bread; an IV-appropriate near-singular guard would need a different mechanism. | `had.py` | structural-rank-guard / 2026-06-28 |
 | **`ImputationDiD` SE vcov is already rank-guarded upstream.** Excluded from the structural rank-guard sweep: the lead/effect vcov comes from `solve_ols(..., return_vcov=True, rank_deficient_action=...)` at the OLS fit (`imputation.py:~2316`), which already drops rank-deficient columns. The only raw inverse (`solve(V_gamma, gamma)`, `imputation.py:~2530`) is the pretrends **Wald F-test statistic** with a safe `NaN` fallback — a test statistic, not a sandwich bread — so there is no garbage-SE exposure. No structural rank-guard needed. | `imputation.py` | structural-rank-guard / 2026-06-28 |
+| **TWFE HC2/HC2-BM full-dummy dedup: drift-prone duplication already resolved by the shared builder; full delegation waived.** The former Actionable row (origin: follow-up review, citing pre-#655 line numbers) asked to extract a shared dummy-construction helper or delegate TWFE's HC2/HC2-BM path to DiD's `fixed_effects=` branch. The shared-helper half SHIPPED in #655: both sites now delegate dummy construction, drop-first convention, FE column naming, and the duplicate-term backstop to the single `build_fe_dummy_blocks` (`utils.py`) + `validate_design_term_names` implementation (`twfe.py::fit` full-dummy branch; `estimators.py::DifferenceInDifferences.fit` `fixed_effects=` branch) — the FE-naming / survey-behavior drift risk the row targeted is gone. What remains per site is ~4 lines of genuinely estimator-specific design-matrix assembly (TWFE stacks `const`/`ATT`/covariates; DiD stacks its formula terms), which is not drift-prone duplication. The remaining full-delegation option — routing `TWFE.fit` through DiD machinery with TWFE-specific cluster-default threading — would touch TWFE's user-visible result surface (coefficient-dict keys, cluster-label conventions, warning text) for near-zero residual benefit; waived on cost/benefit. | `twfe.py::fit`, `estimators.py::DifferenceInDifferences.fit`, `utils.py::build_fe_dummy_blocks` | #655 / 2026-07-10 |
 | **Survey TSL SE intentionally counts genuine-subpopulation zero-weight PSUs (matches R, NOT a bug).** Re-examined the former "count only positive-weight PSUs in the correction" item (origin PR-B). `_compute_stratified_psu_meat`'s finite-sample correction `(1-f_h)·n_{PSU,h}/(n_{PSU,h}-1)` and PSU-mean centering keep zero-weight PSUs — this is the **full-design domain-estimation convention** (Lumley 2004 §3.4; R `survey::svyrecvar(subset())`), already documented in REGISTRY § "Subpopulation Analysis" (the survey-vcov path deliberately differs from the positive-weight invariance applied *outside* it). The ATT is exactly invariant; the SE is intentionally NOT invariant to genuine-subpopulation zeroing (it *should* differ from a naive physical subset — that is the whole point of `subpopulation()`). Repro (`scratchpad`): zeroing a full PSU vs physically dropping it differs ~5e-3 rel — the Lumley-correct gap, and R's `svyrecvar(subset())` produces the matching SE (only `df` differs; see the § "Subpopulation Analysis" Deviation note). The only truly invariance-violating shape — appending *synthetic new* all-zero PSUs — does not arise in any estimator path (real padding reuses existing PSU labels and is already bit-invariant, or is genuine domain estimation via `prep.py`'s zero-padded full-design cell variance). "Fixing" the meat to positive-weight-only would break the documented Lumley/R parity. Regression-locked by `tests/test_survey.py::TestZeroWeightPsuConventionWaiver`. | `survey.py` (`_compute_stratified_psu_meat`) | PR-B / 2026-06-30 |
 
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