perf(lazy): sink key predicates below mean, min and max aggregations - #429
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The conservative fence on mean/min/max was argued from an all-null group's null cell flipping the stitched column's backend - a difference column equality no longer observes. The crossing now carries its own argument: a predicate that reads only bare keys gives every row of a group the same verdict, so the group set and every group's cells are fixed by the crossing conditions themselves, and these ops are pure functions of their group's cells - per-group values, null-ness, and raised errors all unchanged. The row_stable-operand demand stays (a value-raising operand still fences), and std/var/median/first/last remain fenced as a conservative bound, pinned by tests.
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Unfreezes the first deferred optimizer item. The fence on
mean/min/maxwas argued from an all-null group's null cell flipping the stitched column's backend — a difference column equality no longer observes, so the old argument is void rather than merely weakened. The crossing now carries its own group-invariance argument: a predicate reading only bare keys gives every row of a group the same verdict, so the group set and every group's cells are fixed, and these ops are pure functions of their group's cells. Therow_stable-operand demand stays (a value-raising operand still fences), andstd/var/median/first/lastremain fenced, pinned by tests. Behaviour-verified against the eager chain on the all-null-group shape the old fence was argued from. 1449 tests green, coverage fully covered, mbti unchanged.