Refactor: move function _cast_and_arrow to schema.py

src/sp2xr/schema.py

@@ -174,3 +174,16 @@ def enforce_schema(
         ]
 
     return ddf
+
+
+def _cast_and_arrow(pdf: pd.DataFrame) -> pd.DataFrame:
+    """
+    Cast every column in *this* partition to the canonical dtype
+    (or DEFAULT_FLOAT), then switch the frame to pyarrow-backed dtypes.
+    Works for both concentration and histogram dataframes.
+    """
+    cast_map = {
+        col: CANONICAL_DTYPES.get(col, DEFAULT_FLOAT) for col in pdf.columns  # fallback
+    }
+    pdf = pdf.astype(cast_map, copy=False)
+    return pdf.convert_dtypes(dtype_backend="pyarrow")