Final P3 cleanup: remove unreachable TWFE conley auto-cluster + fix resolve_vcov_type docstring

Address two P3 documentation/maintenance findings from CI Codex review of
PR #411 R-final:

- twfe.py:234-240: an `elif self.vcov_type == "conley"` branch in the
  cluster-resolution chain disabled the auto-cluster default for Conley.
  Unreachable behind the unconditional Conley reject at twfe.py:146-163.
  Removed.
- linalg.py::resolve_vcov_type docstring: the "Rules" section and
  Returns docstring listed only `{"classical", "hc1", "hc2", "hc2_bm"}`,
  omitting `"conley"` even though `_VALID_VCOV_TYPES` includes it and
  the runtime path already accepts it. Updated both surfaces; the source
  of truth pointer (`_VALID_VCOV_TYPES`) is now named explicitly so
  future enum additions don't need a docstring sweep.

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

Author: igerber

diff_diff/linalg.py

@@ -1112,8 +1112,9 @@ def resolve_vcov_type(
 
     - If ``vcov_type`` is ``None``: map ``robust=True`` to ``"hc1"`` and
       ``robust=False`` to ``"classical"``.
-    - If ``vcov_type`` is supplied: it must be one of
-      ``{"classical", "hc1", "hc2", "hc2_bm"}``.
+    - If ``vcov_type`` is supplied: it must be one of the values in the
+      module-level ``_VALID_VCOV_TYPES`` set, namely
+      ``{"classical", "hc1", "hc2", "hc2_bm", "conley"}``.
     - If ``robust=False`` is supplied together with a non-``"classical"`` ``vcov_type``,
       raise ``ValueError`` - the combination is ambiguous.
 
@@ -1127,7 +1128,7 @@ def resolve_vcov_type(
     Returns
     -------
     str
-        One of ``"classical"``, ``"hc1"``, ``"hc2"``, ``"hc2_bm"``.
+        One of ``"classical"``, ``"hc1"``, ``"hc2"``, ``"hc2_bm"``, ``"conley"``.
 
     Raises
     ------

diff_diff/twfe.py

@@ -231,13 +231,6 @@ def fit(  # type: ignore[override]
             # Explicit classical + analytical inference: drop the auto-cluster
             # so the validator doesn't reject ``cluster_ids + classical``.
             cluster_var = None
-        elif self.vcov_type == "conley":
-            # Conley + TWFE: disable the auto-cluster default. Conley + cluster
-            # is deferred to Phase 2; the user wants spatial-HAC at the unit
-            # level via the kernel, not cluster-robust at the unit level. The
-            # explicit-cluster case is rejected upstream; reaching here means
-            # cluster=None.
-            cluster_var = None
         else:
             cluster_var = unit