better-data: Adding a validation rule

For library maintainers introducing a new built-in validation rule (Rule\CreditCard, Rule\PhoneE164, Rule\StrongPassword, etc.). Rules are tiny, pure, attribute-decorated classes that the validator iterates per field. The contract is small but precise — getting it wrong (throwing instead of returning, applying to nulls, embedding side effects) breaks compositional rules and surfaces messy errors to consumers.

Misconception this skill corrects

"I'll throw an exception with the validation error message inside check() — easier than a return-string protocol."

The contract at src/Validation/Rule.php:25-28 is:

interface Rule
{
    public function check(mixed $value, string $fieldName, DataObject $subject): ?string;
}

null = pass, short error string = fail. Throwing is the engine's prerogative, not an individual rule's. The BuiltInValidator (src/Validation/BuiltInValidator.php) iterates rules across many fields and accumulates errors; an exception would short-circuit the entire validation pass and return a single error instead of the complete failure list — which is what ValidationResult is for.