[PHP8.6] Readonly property default values

[lisachenko]

Part of epic #219.

Feature

PHP 8.6 (Readonly Property Defaults RFC) lifts the restriction that readonly properties cannot declare default values:

final class CreateBooksTable implements MigratesUp
{
    public readonly string $name = '2026-01-01_create_books_table';
}

Scope

• Ensure parsing a readonly property with a default value works end-to-end (PHP-Parser already parses the syntax; previously it was a compile error so the combination is untested here).
• Verify/fix the affected API surface in src/ReflectionProperty.php and Traits/ReflectionClassLikeTrait.php:
  • hasDefaultValue() / getDefaultValue() on readonly properties
  • ReflectionClass::getDefaultProperties() including readonly defaults
  • interaction with promoted readonly parameters and typed properties
• Check native 8.6 semantics for whether such a property still reports uninitialized-related behavior consistently and mirror it.
• Stub file with readonly defaults (plain, typed, const-expression defaults resolved by NodeExpressionResolver, class constants, enum cases as defaults) + parity tests guarded for 8.6 runtime, direct assertions on 8.5.

Acceptance criteria

• Static results match native reflection on 8.6 for the whole stub matrix.
• PHPUnit green on 8.5, PHPStan level 10 clean.