Summary
Window support is limited to rowNumber/rank/denseRank, which already build OVER (PARTITION BY ... ORDER BY ...). Generalize into a window(fn, { partitionBy, orderBy, frame }) or add lag/lead/sumOver/avgOver helpers reusing the existing overParts builder.
Effort: M (OVER-clause assembly already present). Location: src/client.ts:~2889-2924. Found via internal code audit.