sharedGroupCapacity(parentGroupIds: readonly number[],childGroupIds: readonly number[],staticCapByGroupId: ReadonlyMap<number, number>,remainingByGroupId: ReadonlyMap<number, number>,): SharedGroupCapacity
Build the SharedGroupCapacity for a parent/child pair. The two are co-grouped when their group sets meet in at least one capped group.
Both facts come from the SAME shared groups, and are the tightest value over those — never the child's tightest group overall. That is what lets a date-less surface reject a share too small to hold both, even when a daily child's per-date remaining is unknown.