twfe: scope coefficients-dict broadening to HC2/HC2-BM only

CI review (R7) flagged a P2 scope creep in the R1 coefficients-dict
fix: I built `_twfe_var_names` on BOTH the full-dummy and within-
transform branches, which silently broadened
`result.coefficients` on HC1/classical/Conley paths from
`{"ATT": att}` to `{"const": c, "ATT": att, ...covariates}`. That's
a user-visible API change on unchanged TWFE paths that wasn't
documented in CHANGELOG/REGISTRY or regression-tested.

Per the reviewer's recommendation, restoring the historical
`{"ATT": att}` contract on within-transform paths by setting
`_twfe_var_names = None` on the else branch (the fallback at the
DiDResults construction site handles None via the existing
`{"ATT": float(att)}` literal). Only the HC2/HC2-BM full-dummy path
now broadens the dict — which is what the REGISTRY/CHANGELOG
surface-change disclosure documents, and what the alignment-invariant
test and full-surface regression test pin.

Verified end-to-end: hc1/classical → `{'ATT'}`; hc2/hc2_bm →
`{'const', 'ATT', '_fe_unit_*', '_fe_time_*'}`.

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

Author: igerber

diff_diff/twfe.py

@@ -359,9 +359,13 @@ def fit(  # type: ignore[override]
                     X_list.append(data_demeaned[f"{cov}_demeaned"].values)
             X = np.column_stack([np.ones(len(y))] + X_list)
             df_adjustment = n_units + n_times - 2
-            # Within-transform path: FE dummies are NOT in X (they're absorbed
-            # by demeaning). var_names cover the visible columns only.
-            _twfe_var_names = ["const", "ATT"] + list(covariates or [])
+            # Within-transform path: preserve the historical
+            # `{"ATT": att}` user-facing `result.coefficients` contract.
+            # Broadening this dict here would silently change the
+            # API surface on HC1 / classical / Conley fits — the
+            # full-dummy `_twfe_var_names` exposure is scoped to the
+            # HC2 / HC2-BM paths only (the documented surface change).
+            _twfe_var_names = None
 
         # ATT is the coefficient on treatment_post (index 1) on both branches.
         att_idx = 1