Add Machine.IsTerminal(name string) bool

pkg/statemachine.Machine already exposes State(name) (State, bool), which returns a full State struct (Name, Terminal, Failure) plus an ok bool. Any caller that only wants to know "is this state terminal?" has to call State, check ok, then check .Terminal -- three steps for a one-fact question. This is a small ergonomic gap in the reusable library itself.

There is no one-line way for a caller of pkg/statemachine to ask "is this state terminal?" -- they must call State, check ok, then check .Terminal, three steps for a one-fact question.

m, _ := statemachine.New("x").State("a", statemachine.Terminal()).State("b").Build() m.IsTerminal("a") // should be true m.IsTerminal("b") // should be false m.IsTerminal("no-such") // should be false, not panic

This is a small ergonomic gap in the reusable library itself -- every future caller of pkg/statemachine (not just internal/payment) hits this same three-step dance if they ever want to decide something based on terminality (e.g. "should the caller bother persisting a new updated_at-adjacent audit trail entry, or scheduling a timeout, once a transition lands on a terminal state?").

Look at how Machine.State already does the m.states[name] lookup and returns the zero value + false on a miss. IsTerminal needs the same lookup, just condensed to a single bool. You do not need to change State, stateDef, or any Builder code -- this is purely a new read-only method on Machine.