UnknownHintTarget typedef
A sealed-off step: it references a targetId absent from the registry but
with close candidates (distance ≤ 2) — almost certainly a typo.
typoId is the offending id itself (a step may spotlight several
targets — HintStep.targetIds); the step is skipped wholesale.
Implementation
typedef UnknownHintTarget = ({
HintStep step,
int index,
String typoId,
List<String> candidates
});