Guard<T> as a @FunctionalInterface with default methods

Context

Guard<T> needs to be composable (and, or, negate) while also allowing concise definition via lambdas or method references.

Decision

Guard<T> is a @FunctionalInterface whose single abstract method is check(T value). All composition logic is implemented as default methods on the interface.

Consequences

Positive:

• The user can define a guard with a lambda: Guard<String> notBlank = s -> Validated.valid(s).
• Composition (and, or, negate, contramap) is available without inheritance.
• No state: each guard is a pure function.

Negative / tradeoffs:

• default methods cannot be final — any implementor could override the composition logic (though that would be a misuse of the API).
• The interface grows in surface area as more composition operators are added.

Alternatives considered

• Abstract class AbstractGuard<T>: allows final on composition methods but prevents lambda usage and multiple inheritance.
• Concrete class Guard<T> with a Function field: more rigid, not extensible as an interface.