feat(arrow): opt-in coalesce of heterogeneous temporal lists to Timestamp(ns)

KDB returns mixed-typed columns (most commonly: temporal nulls of
varying precision -- 0Np, 0Nz, 0Nd -- interleaved) as q general lists.
The faithful Arrow projection emits a `Union` of the per-element
DataTypes. Polars and most DataFrame consumers reject `Union` outright,
making such columns unusable downstream without a re-roundtrip dance.

Add `HeterogeneousListMode::CoalesceTemporals` (off by default) on
`ProjectionOptions`. When set, `project_heterogeneous_list` checks
whether every arm is temporal (Timestamp, Date32, Date64, Time32,
Time64, Duration) and, if so, casts each child to
`Timestamp(Nanosecond, None)` via `arrow_cast::cast`, concatenates, and
emits a flat array. Any non-temporal arm or cast error falls back to
the existing Union path, so the flag is safe to enable globally.

Plumbed through the Python `DecodeOptions` API as
`with_coalesce_temporals(bool)` with matching getter and pyi stub. The
default stays `False`; users opt in when they know the consumer
(Polars) can't handle Union and accept the lossy precision promotion.

Tests cover (a) default-Union, (b) all-temporal coalesce, and
(c) non-temporal fallback to Union.

python/qroissant/_config.pyi

@@ -260,6 +260,14 @@ class DecodeOptions:
     def treat_infinity_as_null(self) -> bool:
         """Whether ±∞ sentinels are mapped to ``None`` in Arrow arrays."""
         ...
+    @property
+    def coalesce_temporals(self) -> bool:
+        """Whether heterogeneous all-temporal lists are flattened to ``Timestamp(ns)``
+        instead of being emitted as Arrow ``Union``.  Use this when the
+        downstream consumer (e.g. Polars) cannot ingest ``Union`` types.
+        Lossy: mixed precisions are promoted to nanoseconds.
+        """
+        ...
 
 
 class DecodeOptionsBuilder:
@@ -303,6 +311,15 @@ class DecodeOptionsBuilder:
     def with_treat_infinity_as_null(self, value: bool, /) -> DecodeOptionsBuilder:
         """Set whether ±∞ sentinels are mapped to ``None`` in Arrow arrays."""
         ...
+    def with_coalesce_temporals(self, value: bool, /) -> DecodeOptionsBuilder:
+        """Set whether heterogeneous all-temporal lists are flattened to
+        ``Timestamp(ns)`` instead of being emitted as Arrow ``Union``.
+
+        Set to ``True`` for consumers (e.g. Polars) that reject Arrow union
+        types.  Lossy: mixed precisions are promoted to nanoseconds.  Default
+        ``False`` (faithful ``Union`` representation).
+        """
+        ...
     def build(self) -> DecodeOptions:
         """Finalize the builder into an immutable :class:`DecodeOptions` instance."""
         ...