NotCoercible
in package
A payload-free enum used as {@see Coercion::tryCoerce()}'s "no applicable coercion" sentinel, comparable via `!==`/`instanceof` -- a coerced value can legitimately be `null` (a value coerced toward a `nilable` field, say), so `null` itself can't double as "coercion didn't apply" the way JS's own `tryCoerce` uses `undefined`.