Add Builder.MustBuild() *Machine

Builder.Build() returns (*Machine, error). Every current caller in this repo (payment.BuildMachine, the library's own tests, the order-fulfillment example) either propagates that error or handles it explicitly -- reasonable for a caller that takes user-configurable durations and can genuinely fail at runtime. But a caller with a fully static, compile-time-fixed table has no real recovery path for a "build error" other than to panic anyway -- except right now they'd have to write that panic themselves.

There is no Builder.MustBuild() convenience -- every caller with a statically-known-correct table has to write its own three-line "call Build, panic on error" wrapper.

machine := statemachine.New("x").State("a").State("b", statemachine.Terminal()). Transition("a", "go", "b", nil, nil).MustBuild() // currently panics even though this table is perfectly valid

This is a small but genuine ergonomic gap in the reusable library: the "panic on a startup-time, should-never-actually-happen error" pattern is common enough (regexp.MustCompile, template.Must) that not offering it forces every such caller to reimplement the same three lines.

Look at how Build() is implemented -- MustBuild should call it and panic on a non-nil error, returning the *Machine otherwise. This is a thin wrapper; don't duplicate any of Build's validation logic.