Stella McCartney’s stock-in-trade is offering up pieces that strike the right balance between being effortless to wear, but actually additive to one’s wardrobe. There’s often a tomboyish influence that makes her women’s line always seem just a bit cooler than the rest on the fashion block. It wasn’t a total surprise that she announced this summer that she was working on a menswear collection. The first sneak peek of McCartney’s new line was seen at the Spring/Summer 2017 fashion show in Paris last month, when her husband Alasdhair Willis wore a four-button double-breasted jacket and matching trousers. The first impression conveyed the mood of her menswear: streamlined, uncomplicated, and…