fix: enforce readonly declaration and inheritance rules

[AlessioGiacobbe]

Readonly declaration rules were unchecked for ZendVM-backed classes (checks existed only in the Native-class branch, which rejects readonly wholesale): public readonly int $x = 5; (Zend: "cannot have default value"), untyped readonly ("must have type"), and static readonly all compiled, for declared and promoted properties and via readonly class — each rule probed for exact Zend behavior; promoted readonly params keep their legal parameter defaults.

Readonly-class inheritance was also not sealed: a non-readonly class extending a readonly one (and the reverse) compiled; both directions now fail with Zend's messages.

One pre-existing fixture (inheritance_error_prop_readonly.php) used readonly int $x = 2, which Zend itself rejects before the inheritance error under test; the default was dropped so the fixture still exercises the mismatch.

Part of the split of #39.

[matyhtf]

The focused readonly tests pass (11/11), but three readonly class contracts still compile:

1. Trait property origin must retain readonlyness:

trait T { public int $value; }
readonly class C { use T; }

Zend: Readonly class C cannot use trait with a non-readonly property T::$value. The current composition treats the consuming class flag as if it converted the trait property to readonly. It must reject the mismatch; add positive coverage for a trait property explicitly declared readonly.

2. Internal parents are skipped by the inheritance check:

readonly class C extends ArrayObject {}

Zend: Readonly class C cannot extend non-readonly class ArrayObject. Query the internal parent through ReflectionClass::isReadOnly() (or equivalent metadata) so the rule applies to both TypePHP and internal parents.

3. #[AllowDynamicProperties] is forbidden on readonly classes:

#[AllowDynamicProperties]
readonly class C { public function __construct(public int $value) {} }

Zend rejects it, while TypePHP compiles it. tests/compiler/class/readonly-class.phpt currently contains this invalid combination and should be corrected when adding the negative test.

Please cover and reject all three before merge.

[AlessioGiacobbe]

All three covered, rebased on current master:

1. Trait properties — rejected during composition with Zend's message ("Readonly class C cannot use trait with a non-readonly property T::$value"). Placement detail: Zend names the directly used trait for nested traits (probed with an Inner→Outer chain — it reports Outer::$v), so the check runs in the composition pass where that name is in hand rather than off the origin attribute. Static trait properties are rejected too (they can never be readonly — probed); an explicitly-readonly trait property still composes (positive test).
2. Internal parents — the inheritance check no longer skips them: an internal-parent branch queries host ReflectionClass::isReadOnly(), two-directional in principle (today's only internal readonly classes, BcMath\Number and Dom\NamespaceInfo, are both final, so only the readonly-child direction is constructible — reflection keeps the reverse correct regardless). readonly class C extends ArrayObject {} now fails with Zend's message.
3. #[AllowDynamicProperties] — rejected on readonly classes with Zend's wording, and the invalid combination sitting in the pre-existing readonly-class.phpt is fixed (body re-validated under Zend, output unchanged); the negative test lives in the phpunit suite per the repo's convention for front-end rejections.

5 new fixtures, 21/21 with the related suites; sweep and full suite identical to base.