toBigInt method
Transform the value to a BigInt.
Implementation
NumberBuilder toBigInt() {
// Represent BigInt coercion using double_ slot to avoid new enum value (keeps simplicity)
if (chain.coercedToDouble) return NumberBuilder(chain: chain);
chain.setTransform(CoercionKind.double_, (child) => tr.toBigInt(child));
return NumberBuilder(chain: chain);
}