OnPair: decode straight into the builder's byte storage - #9100

The base and head of this comparison were measured with different runner settings, so their benchmark values are not directly comparable.

What changed between base and head:

• CodSpeed runner v5 changed how benchmarks are measured (base 4.13.0 → head 5.0.1). View release notes
• Cycle estimation (off → on) changes how benchmark performance is calculated. Learn more about cycle estimation

Re-run the base with the same settings to get a valid performance comparison.

_{Comparing rk/onpair-decode-into-builder (ee24c54) with rk/zstd-append-to-builder (3f1ff67)¹}