Reduce allocations in approx_percentile QuantileSummaries merge and flush paths

[andygrove]

This is a follow-up to a review comment on #4801 (#4801 (comment)).

What is the problem the feature request solves?

The native QuantileSummaries port behind approx_percentile / percentile_approx allocates more than necessary on its hot paths:

• QuantileSummaries::merge takes &self / &other and allocates a fresh summary, so merge_batch reallocates on every incoming digest.
• with_head_buffer_inserted rebuilds the whole sampled vector on every flush.

These are not correctness issues, and results are bit-identical to Spark, but they show up as avoidable allocation churn when many partial digests are merged or when a single accumulator ingests many batches.

Describe the potential solution

• A double-buffer for the flush so with_head_buffer_inserted reuses a scratch Vec instead of allocating a new sampled each time.
• A consuming merge(self, other: &Self) -> Self (or an in-place merge_into(&mut self, other: &Self)) so merge_batch does not reallocate per digest.

Both changes must preserve the existing bit-for-bit results; the load-bearing invariants are documented at the top of native/spark-expr/src/agg_funcs/quantile_summaries.rs.

Additional context

Worth doing only if a profile shows this path is hot. Noted during review of #4801 as an explicit out-of-scope follow-up.