You land in Rome at 11 a.m. wearing a sweater, spend the afternoon sweating through it, then walk home in cold rain after dinner. That is the shoulder-season problem in one day. Spring and autumn city breaks punish single-purpose clothes, bulky coats, and shoes that cannot handle wet pavement. After years of five-day trips with one carry-on, I now pack around a simple rule: three layers, two pairs of shoes, and clothes that work in at least three outfits. This list assumes mild but changeable weather, with daytime temperatures from 50 to 68 degrees Fahrenheit and occasional rain. What Shoulder…