Lululemon Athletica’s shares spiked Friday after the company’s CEO Calvin McDonald announced his resignation amid declining US sales, with its founder putting the board on notice about a potential proxy battle.After seven years at the helm, McDonald is stepping down on Jan. 31 and will remain as an advisor to the company through March, he said on a Thursday earnings call discussing two straight quarters of decreasing North America sales.“We agreed that the timing is right for a change,” McDonald said of his ...