sharedGroupRemaining(parentGroupIds: readonly number[],childGroupIds: readonly number[],remainingByGroupId: ReadonlyMap<number, number>,): number | undefined
The remaining spots of the capped group a parent and one of its children
share, or undefined when they share no capped group. Such a parent and
child take two group spots per order, so a caller must reason about the
combined demand, not each row alone.
The value is the tightest SHARED group's remaining, never the child's tightest group overall — a child that is also in a tighter unshared group must not drag this down to an unrelated cap. Discovery and the booking-page quantity ceiling both read it, so the two surfaces cannot disagree.