feat(array): builders no longer canonicalize their children

Nested builders used to push every appended child array through
`append_to_builder`, which decoded it into the child's canonical builder.
That work is wasted: `Canonical` only promises a canonical *top level*, so
struct fields, list elements and extension storage are free to stay
compressed.

Introduce `ChildBuilder`, which accumulates a child as a `Vec<ArrayRef>` of
chunks plus a scalar builder for the values that cannot come from an array,
and stitches them into a `ChunkedArray` on `finish` when more than one chunk
accumulated. `StructBuilder`, `ListBuilder`, `ListViewBuilder`,
`FixedSizeListBuilder` and `ExtensionBuilder` now hold their children this
way.

However short the appended array, it becomes a chunk. Deciding on the caller's
behalf that its values are cheaper copied than referenced would be guessing at
a boundary only the caller can see, and a caller that wants them copied has
`append_scalar`. A child is therefore chunked on exactly the boundaries it was
appended on.

Signed-off-by: Claude <noreply@anthropic.com>
Signed-off-by: Robert Kruszewski <robert@spiraldb.com>
Signed-off-by: Robert Kruszewski <github@robertk.io>

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: robert3005

encodings/sparse/src/canonical.rs

@@ -585,6 +585,7 @@ mod test {
     use vortex_array::IntoArray;
     use vortex_array::VortexSessionExecute;
     use vortex_array::arrays::BoolArray;
+    use vortex_array::arrays::Chunked;
     use vortex_array::arrays::DecimalArray;
     use vortex_array::arrays::FixedSizeListArray;
     use vortex_array::arrays::ListArray;
@@ -593,6 +594,7 @@ mod test {
     use vortex_array::arrays::StructArray;
     use vortex_array::arrays::VarBinArray;
     use vortex_array::arrays::VarBinViewArray;
+    use vortex_array::arrays::chunked::ChunkedArrayExt;
     use vortex_array::arrays::listview::ListViewArrayExt;
     use vortex_array::arrays::listview::ListViewArraySlotsExt;
     use vortex_array::assert_arrays_eq;
@@ -1338,6 +1340,50 @@ mod test {
         Ok(())
     }
 
+    /// Nested builders chunk a child on the boundaries it is appended on, so the number of appends
+    /// canonicalization makes is now visible in the elements child. A run of consecutive patches
+    /// and the gap after it should cost one chunk each, not one chunk per row.
+    #[test]
+    fn test_sparse_list_chunks_elements_per_run_not_per_patch() -> VortexResult<()> {
+        let mut ctx = SESSION.create_execution_ctx();
+
+        const PATCHES: usize = 100;
+        let patches_u32 = u32::try_from(PATCHES).vortex_expect("fits in u32");
+        let patches_i32 = i32::try_from(PATCHES).vortex_expect("fits in i32");
+
+        // `PATCHES` single-element lists, patched onto rows 0..PATCHES of a 2 * PATCHES-row array,
+        // so there is exactly one patch run followed by exactly one gap.
+        let patch_values = ListViewArray::new(
+            PrimitiveArray::from_iter(0..patches_i32).into_array(),
+            PrimitiveArray::from_iter(0..patches_u32).into_array(),
+            PrimitiveArray::from_iter(std::iter::repeat_n(1u32, PATCHES)).into_array(),
+            Validity::AllValid,
+        )
+        .into_array();
+
+        let indices = PrimitiveArray::from_iter(0..patches_u32).into_array();
+        let fill = Scalar::from(Some(vec![-1i32]));
+        let sparse = Sparse::try_new(indices, patch_values, 2 * PATCHES, fill)?.into_array();
+
+        let actual = sparse.execute::<ListViewArray>(&mut ctx)?;
+        assert_eq!(
+            actual.elements().as_::<Chunked>().nchunks(),
+            2,
+            "expected one chunk for the patch run and one for the gap",
+        );
+
+        let expected_lists = (0..patches_i32)
+            .map(|i| Some(vec![i]))
+            .chain(std::iter::repeat_n(Some(vec![-1i32]), PATCHES));
+        let expected = ListArray::from_iter_opt_slow::<u32, _, _>(
+            expected_lists,
+            Arc::new(PType::I32.into()),
+        )?;
+        assert_arrays_eq!(actual, expected, &mut ctx);
+
+        Ok(())
+    }
+
     /// Patches on consecutive rows are appended as one slice of the patch array, so this covers the
     /// run arithmetic together with everything that has to survive it: a null patch inside a run,
     /// patch views that overlap and are out of order, runs separated by gaps, and a trailing gap.

vortex-array/src/aggregate_fn/fns/uncompressed_size_in_bytes/mod.rs

@@ -329,6 +329,7 @@ mod tests {
 
     use crate::ArrayRef;
     use crate::IntoArray;
+    use crate::RecursiveCanonical;
     use crate::VortexSessionExecute;
     use crate::aggregate_fn::Accumulator;
     use crate::aggregate_fn::AggregateFnVTable;
@@ -368,14 +369,21 @@ mod tests {
     use crate::validity::Validity;
 
     fn materialized_uncompressed_size_in_bytes(array: &ArrayRef) -> u64 {
+        let mut ctx = array_session().create_execution_ctx();
         let mut builder = builder_with_capacity(array.dtype(), array.len());
         array
-            .append_to_builder(
-                builder.as_mut(),
-                &mut array_session().create_execution_ctx(),
-            )
+            .append_to_builder(builder.as_mut(), &mut ctx)
             .vortex_expect("appended");
-        builder.finish().nbytes()
+
+        // A builder keeps its children in whatever encoding they were appended in, so the bytes of
+        // what it finishes only stand in for the uncompressed size once the whole tree is decoded.
+        builder
+            .finish()
+            .execute::<RecursiveCanonical>(&mut ctx)
+            .vortex_expect("recursively canonicalized")
+            .0
+            .into_array()
+            .nbytes()
     }
 
     fn aggregate(array: &ArrayRef) -> VortexResult<u64> {