dataset: add fallback overload for get_surf's dynamic hemi/merge callers

Ported from 9abb080 (types-data branch) / bb81c50a (types-easy); the rest of
that change (retyping Database.auxfile, fixing get_cache's hashlib bug) is
owned by PR 4 (5b3496d on types-01-03-04-infra-geometry-database), not here.

Author: kroq-gar78

cortex/dataset/dataset.py

@@ -166,6 +166,11 @@ def get_surf(self, subject: str, type: str, hemi: Literal['both']='both', *, mer
     @overload
     def get_surf(self, subject: str, type: str, hemi: Literal['lh', 'rh'], merge: bool=False, nudge: bool=False) -> tuple[npt.NDArray[np.floating], npt.NDArray[np.integer]]: ...
 
+    # Fallthrough case for callers (e.g. Database.get_surf) forwarding a dynamic
+    # hemi/merge that isn't statically one of the above.
+    @overload
+    def get_surf(self, subject: str, type: str, hemi: Literal['both', 'lh', 'rh']='both', merge: bool=False, nudge: bool=False) -> Union[tuple[tuple[npt.NDArray[np.floating], npt.NDArray[np.integer]], tuple[npt.NDArray[np.floating], npt.NDArray[np.integer]]], tuple[npt.NDArray[np.floating], npt.NDArray[np.integer]]]: ...
+
     def get_surf(self, subject: str, type: str, hemi: Literal['both', 'lh', 'rh']='both', merge: bool=False, nudge: bool=False) -> Union[tuple[tuple[npt.NDArray[np.floating], npt.NDArray[np.integer]], tuple[npt.NDArray[np.floating], npt.NDArray[np.integer]]], tuple[npt.NDArray[np.floating], npt.NDArray[np.integer]]]:
         pts: npt.NDArray[np.floating]
         polys: npt.NDArray[np.integer]