perf: use Narwhals expressions (and stable API) (#528)

* use Narwhals stable api, use expressions

* just use .dt.convert_time_zone

vegafusion-python/vegafusion/runtime.py

@@ -4,7 +4,7 @@
 from types import ModuleType
 from typing import TYPE_CHECKING, Any, Literal, TypedDict, Union, cast
 
-import narwhals as nw
+import narwhals.stable.v1 as nw
 from arro3.core import Table
 
 from vegafusion._vegafusion import get_cpu_count, get_virtual_memory
@@ -566,18 +566,15 @@ def pre_transform_datasets(
                     "transformed data"
                 )
 
-        # Localize datetime columns to UTC, then extract the native DataFrame
-        # to return
+        # Convert to `local_tz` (or, set to UTC and then convert if starting
+        # from time-zone-naive data), then extract the native DataFrame to return.
         processed_datasets = []
         for df in nw_dataframes:
-            for name in df.columns:
-                dtype = df[name].dtype
-                if dtype == nw.Datetime:
-                    df = df.with_columns(
-                        df[name]
-                        .dt.replace_time_zone("UTC")
-                        .dt.convert_time_zone(local_tz)
-                    )
+            df = df.with_columns(
+                nw.col(col).dt.convert_time_zone(local_tz)
+                for col, dtype in df.schema.items()
+                if dtype == nw.Datetime
+            )
             processed_datasets.append(df.to_native())
 
         return processed_datasets, warnings