addStake method
Add stake to a validator (create if missing).
Implementation
void addStake(String id, double amount) {
if (amount <= 0) throw ArgumentError('amount>0');
final idx = validators.indexWhere((v) => v.id == id);
if (idx == -1) {
validators.add(Validator(id, amount));
} else {
validators[idx].stake += amount;
}
}