Several major firms that profit from low wages in Bangladesh have signed onto a plan requiring them to help pay for fire safety and building improvements at factories. Those who backed the plan Monday include Sweden-based H&M, the largest purchaser of Bangladeshi garments; British firms Primark and Tesco; and Spain’s Inditex, which owns the Zara chain. The legally binding contract requires companies to conduct independent safety inspections, pay for repairs and end dealings with factories that refuse to improve safety. The apparel company PVH, which agreed to sign on last year, is the only U.S. company to embrace the plan. Wal-Mart — the second-largest producer of clothing in Bangladesh — and the Gap have not signed on.