perf: reuse quantile summary buffers during merge

[peterxcli]

Which issue does this PR close?

Closes #4874.

Rationale for this change

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.

What changes are included in this PR?

• Reuse scratch buffers in QuantileSummaries during flush, compression, and merge.
• Change summary merging to update in place while preserving the first-digest move fast path.
• Update memory accounting and add a regression test for buffer reuse.

How are these changes tested?

benchmark

• flamegraph:
• source:
  • baseline-nativ-bytes.html
  • patch-native-bytes.html

result:

• merge_batch: (15GB -> 11GB)
  • upstream:
  • pr:
• with_head_buffer_inserted: (10GB -> 1GB)
  • upstream:
  • pr: