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.

crates/qroissant-python/src/serde.rs

@@ -3,6 +3,7 @@ use std::sync::Arc;
 use pyo3::prelude::*;
 use pyo3::types::PyAny;
 use pyo3::types::PyBytes;
+use qroissant_arrow::HeterogeneousListMode;
 use qroissant_arrow::ListProjection;
 use qroissant_arrow::ProjectionOptions;
 use qroissant_arrow::StringProjection;
@@ -56,6 +57,11 @@ pub fn decode_options_to_proj_opts(opts: Option<&DecodeOptions>) -> Arc<Projecti
             crate::types::UnionMode::Dense => qroissant_arrow::UnionMode::Dense,
             crate::types::UnionMode::Sparse => qroissant_arrow::UnionMode::Sparse,
         },
+        heterogeneous_list_mode: if opts.coalesce_temporals_value() {
+            HeterogeneousListMode::CoalesceTemporals
+        } else {
+            HeterogeneousListMode::Union
+        },
         treat_infinity_as_null: opts.treat_infinity_as_null(),
         parallel: opts.parallel_value(),
         assume_symbol_utf8: opts.assume_symbol_utf8_value(),

crates/qroissant-python/src/types.rs

@@ -817,6 +817,7 @@ pub struct DecodeOptions {
     validate_compressed_trailing_bytes: bool,
     temporal_nulls: bool,
     treat_infinity_as_null: bool,
+    coalesce_temporals: bool,
 }
 
 #[pymethods]
@@ -875,6 +876,11 @@ impl DecodeOptions {
     fn get_treat_infinity_as_null(&self) -> bool {
         self.treat_infinity_as_null
     }
+
+    #[getter]
+    fn coalesce_temporals(&self) -> bool {
+        self.coalesce_temporals
+    }
 }
 
 impl Default for DecodeOptions {
@@ -890,6 +896,7 @@ impl Default for DecodeOptions {
             validate_compressed_trailing_bytes: true,
             temporal_nulls: true,
             treat_infinity_as_null: false,
+            coalesce_temporals: false,
         }
     }
 }
@@ -926,6 +933,10 @@ impl DecodeOptions {
     pub(crate) fn assume_symbol_utf8_value(&self) -> bool {
         self.assume_symbol_utf8
     }
+
+    pub(crate) fn coalesce_temporals_value(&self) -> bool {
+        self.coalesce_temporals
+    }
 }
 
 #[pyclass(module = "qroissant", frozen, eq)]
@@ -996,6 +1007,12 @@ impl DecodeOptionsBuilder {
         next
     }
 
+    fn with_coalesce_temporals(&self, value: bool) -> Self {
+        let mut next = self.clone();
+        next.options.coalesce_temporals = value;
+        next
+    }
+
     fn build(&self) -> DecodeOptions {
         self.options.clone()
     }
@@ -1049,6 +1066,11 @@ impl DecodeOptionsBuilder {
     fn treat_infinity_as_null(&self) -> bool {
         self.options.treat_infinity_as_null
     }
+
+    #[getter]
+    fn coalesce_temporals(&self) -> bool {
+        self.options.coalesce_temporals
+    }
 }
 
 #[pyclass(module = "qroissant", frozen, eq)]