feat: Optimize TopK for single primitive column sorts

For ORDER BY on a single primitive column (int, float, date, timestamp,
etc.), bypass Arrow's RowConverter entirely and use inline u128 keys with
order-preserving encoding. This avoids heap allocation per row (Vec<u8>),
batch-level row conversion overhead, and variable-length byte-slice
comparison.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Dandandan