feat: support static interface methods (PHP 8.3+)

[chadmandoo]

What

Adds support for PHP 8.3+ static interface methods — interfaces may now declare public static function members, and implementing classes satisfy them with a static method.

interface Previewable { public static function previews(): array; }
final class C implements Previewable {
    public static function previews(): array { return ['a', 'b', 'c']; }
}
echo implode(',', C::previews()); // a,b,c

Previously this errored at the schema stage: "Static interface methods are not supported yet".

How

• InterfaceInfo gains static_methods (+ static_method_order), recorded apart from instance methods — static dispatch is by class, so they take no vtable slot.
• The interface schema builder routes a static interface method into static_methods (and inherits parent-interface static methods) instead of rejecting it.
• A new validate_interface_static_method enforces conformance: a concrete class must declare a compatible static method (an abstract class may defer); a missing implementation errors with "Class X must implement static interface method I::m".

Verification

• Byte-parity vs PHP 8.5 for a static-interface-method call (C::previews() → identical output).
• Negative case (class omits the static method) correctly errors.
• Instance interfaces unaffected; no regressions in the interface/instanceof suites.