refactor: simplify row_columnar benchmark to address review feedback

Replace magic numbers with named constants (INT64_SIZE,
UNSAFE_ROW_POINTER_SIZE, ARRAY_HEADER_SIZE), unify three struct schema
builders and three row-data structs into single parameterized functions,
and extract duplicated benchmark runner into a shared helper. Reduces
the file from ~845 to ~305 lines.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: andygrove