test: normalize and multiset-compare result sets across engines

[Aybavs]

Renders engine rows and SQLite driver rows into the same canonical cells so results can be compared meaningfully.

• Numerics share one form, so an int and an equal float match — SQLite returns an average as a float where this engine may return an int. Six significant digits absorb last-bit float noise without hiding a real difference (35.0 vs 35.1 still differs).
• Cells carry a type tag, so the text 35 never equals the number 35, and NULL uses a sentinel no generated literal can produce, so the text NULL never reads as NULL.
• Compare sorts both sides, because row order is unspecified without ORDER BY — but duplicate counts still have to match.

Closes #45