Include "conley" in DiD/MultiPeriodDiD vcov_type enum docstrings

Address P3 documentation finding from CI Codex review of PR #411 R-post-rebase.
The leading `vcov_type` enum lists on `DifferenceInDifferences` and
`MultiPeriodDiD` only enumerated `{"classical", "hc1", "hc2", "hc2_bm"}`,
omitting `"conley"` even though the constructor accepts it (sklearn-style
API symmetry — the rejection happens at fit-time, not __init__). The
fit-time rejection note already existed below the enum; this just adds
`"conley"` to the bracketed list and folds the rejection text into the
new bullet so generated docs / IDE help surface the complete public API.

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

Author: igerber

diff_diff/estimators.py

@@ -57,7 +57,7 @@ class DifferenceInDifferences:
         ``vcov_type``: with ``"hc1"`` dispatches to CR1 (Liang-Zeger); with
         ``"hc2_bm"`` dispatches to CR2 Bell-McCaffrey (Pustejovsky-Tipton 2018
         symmetric-sqrt + Satterthwaite DOF).
-    vcov_type : {"classical", "hc1", "hc2", "hc2_bm"}, optional
+    vcov_type : {"classical", "hc1", "hc2", "hc2_bm", "conley"}, optional
         Variance-covariance family. Defaults to the ``robust`` alias.
 
         - ``"classical"``: non-robust OLS SEs, ``sigma_hat^2 * (X'X)^{-1}``.
@@ -69,15 +69,15 @@ class DifferenceInDifferences:
           with ``cluster=``, Pustejovsky-Tipton (2018) CR2 cluster-robust.
           (Note: ``MultiPeriodDiD`` does NOT yet support ``cluster=`` with
           ``"hc2_bm"`` — see ``MultiPeriodDiD`` docstring and REGISTRY.md.)
-
-        ``vcov_type="conley"`` (Conley 1999 spatial-HAC) is **rejected** at
-        fit-time on ``DifferenceInDifferences`` in Phase 1 because DiD is
-        intrinsically a two-period panel design, and Phase 1's cross-
-        sectional Conley does not handle the time dimension. The supported
-        Phase 1 path for Conley is direct ``compute_robust_vcov`` /
-        ``LinearRegression`` on a single-period regression. Phase 2 will
-        add the space-time product kernel (Driscoll-Kraay) and lift the
-        rejection.
+        - ``"conley"``: Conley 1999 spatial-HAC sandwich. **Accepted by the
+          constructor for sklearn-style API symmetry but rejected at
+          fit-time on ``DifferenceInDifferences``** because DiD is
+          intrinsically a two-period panel design, and Phase 1's cross-
+          sectional Conley does not handle the time dimension. The
+          supported Phase 1 path for Conley is direct
+          ``compute_robust_vcov`` / ``LinearRegression`` on a single-period
+          regression. Phase 2 will add the space-time product kernel
+          (Driscoll-Kraay) and lift the rejection.
     alpha : float, default=0.05
         Significance level for confidence intervals.
     inference : str, default="analytical"
@@ -1022,7 +1022,7 @@ class MultiPeriodDiD(DifferenceInDifferences):
         ``TODO.md``; also documented as a Note in
         ``docs/methodology/REGISTRY.md`` under the HeterogeneousAdoptionDiD
         requirements-checklist block.
-    vcov_type : {"classical", "hc1", "hc2", "hc2_bm"}, optional
+    vcov_type : {"classical", "hc1", "hc2", "hc2_bm", "conley"}, optional
         Variance-covariance family. Defaults to the ``robust`` alias.
 
         - ``"classical"``: non-robust OLS SEs, ``sigma_hat^2 * (X'X)^{-1}``.
@@ -1033,15 +1033,15 @@ class MultiPeriodDiD(DifferenceInDifferences):
         - ``"hc2_bm"``: one-way HC2 + Imbens-Kolesar (2016) Satterthwaite DOF
           per coefficient plus a contrast-aware DOF for the post-period-average
           ATT. **Unsupported with** ``cluster=`` — see ``cluster`` above.
-
-        ``vcov_type="conley"`` (Conley 1999 spatial-HAC) is **rejected** at
-        fit-time on ``MultiPeriodDiD`` in Phase 1 because MultiPeriodDiD is
-        intrinsically a multi-period panel estimator and Phase 1's cross-
-        sectional Conley does not handle the time dimension. The supported
-        Phase 1 path for Conley is direct ``compute_robust_vcov`` /
-        ``LinearRegression`` on a single-period regression. Phase 2 will
-        add the space-time product kernel (Driscoll-Kraay) and lift the
-        rejection.
+        - ``"conley"``: Conley 1999 spatial-HAC sandwich. **Accepted by the
+          constructor for sklearn-style API symmetry but rejected at
+          fit-time on ``MultiPeriodDiD``** because MultiPeriodDiD is
+          intrinsically a multi-period panel estimator and Phase 1's
+          cross-sectional Conley does not handle the time dimension. The
+          supported Phase 1 path for Conley is direct
+          ``compute_robust_vcov`` / ``LinearRegression`` on a single-period
+          regression. Phase 2 will add the space-time product kernel
+          (Driscoll-Kraay) and lift the rejection.
     alpha : float, default=0.05
         Significance level for confidence intervals.
     conley_coords, conley_cutoff_km, conley_metric, conley_kernel