aggInto

private fn

void aggInto(string agg,  Kind K,  Build)(ref  GatherCursor!K cursor,
       const  size_t[]  rows,  ref  Build b)

The aggregation arithmetic itself: appends exactly one slot (value or null) into b - the same statements the one-slot kernel has always executed, moved verbatim so the one-slot and batched paths share them (the oracle contract; ADR-2026-10-18-14).

Parameters

`agg`	the aggregation kernel name
`K`	the aggregated kind
`Build`	the builder type (AggBuild!(agg, K))
`cursor`	the column's gather cursor (built once per column by the callers - its constructor allocates, so the batched path must not rebuild it per group)
`rows`	the group's rows, in original order
`b`	the builder the slot appends into