SANDWICH, England—American debutant Collin Morikawa produced an ice-cool final round to win the British Open by two strokes on a blazing hot afternoon at Royal St George’s on Sunday. The 24-year-old began the day a stroke behind Louis Oosthuizen but played picture-perfect golf in a four-under round of 66 to overhaul the South African and resist resurgent fellow American Jordan Spieth in a three-way battle. Morikawa barely missed a fairway all day as he made four birdies and dropped no shots, showing incredible composure late on as Spieth applied pressure down the stretch. He never looked like cracking though and his fourth sub-par round of the week saw him finish on 15-under 265—his remarkable consistency born out by the astonishing statistic of only four bogeys all week. Spieth, who was bidding to win his fourth major and first since the 2017 Open, started his final round badly with two bogeys …