geometry: annotate place_batch return to silence mypy 2.x

CI runs mypy 2.1.0 (strict); the inferred return type of the
conditionally-assigned _place_batch_kernel is Any, which trips
no-any-return on direct return. Bind to a local annotated as np.ndarray
and return that.

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

Author: timodonnell

src/pyconfind/geometry.py

@@ -188,4 +188,5 @@ def place_batch(
     bond = np.ascontiguousarray(bond, dtype=np.float64)
     angle_deg = np.ascontiguousarray(angle_deg, dtype=np.float64)
     dihedral_deg = np.ascontiguousarray(dihedral_deg, dtype=np.float64)
-    return _place_batch_kernel(a, b, c, bond, angle_deg, dihedral_deg)
+    result: np.ndarray = _place_batch_kernel(a, b, c, bond, angle_deg, dihedral_deg)
+    return result