Almost every station on the factory floor of Commonwealth Fusion Systems’ northeast Massachusetts headquarters was bustling on a steamy Friday morning in June. Hard-hatted workers ferried along the components for the company’s first fusion reactor, including giant slabs of magnets that will eventually be joined together in a doughnut-shaped reactor called a tokamak. Construction crews were expanding the facility to make more space for the company, which is racing to be the first private enterprise to prove that nuclear fusion can be a viable source of energy.