Same shape as the FSST change: `append_to_builder` staged the decoded heap in a
temporary buffer before copying it in. `OnPairDecodePlan` hoists the code
window, the dictionary view, the per-row lengths and the total decoded size out
of `onpair_decode_bytes` so the decode can target the builder's spare capacity
via `append_decoded`, leaving one prefix sum over the lengths as the only work
beyond the bulk `try_decode_into`.

`decode_into` is `#[inline(always)]`: `try_decode_into` is generic over the
dictionary view and only specializes its batched copy loop once inlined into the
caller, so leaving the wrapper out of line costs ~2.5x.

Also extends the Arrow export benchmarks to cover OnPair alongside the other
string encodings, plus nullable variants and the two `append_to_builder`
benchmarks that reach an encoding's specialization directly — the Arrow export
cannot stand in for those, since `execute_until` canonicalizes a bare
FSST/OnPair/Zstd root before any builder sees it.

Signed-off-by: Robert Kruszewski <robert@spiraldb.com>