The trial's hours, then credit, with nothing to choose

A sandbox runs on the free trial's hours first and on prepaid credit after, automatically: at create, when a paused sandbox wakes, and while one runs, so a running trial sandbox carries on on credit instead of pausing when the hours run out. A request's funding is accepted and ignored, and code that sets it keeps working. Once an account holds credit, its sandboxes have paid limits and still spend the trial's hours first, and a trial sandbox that turns paid while it runs gets a paid sandbox's disk share at once and, if the trial had slowed it, its full cores back. See included usage.