Language: typed property write enforcement on VM (#169)

[PurHur]

Summary

• Enforce typed property writes on VM: reject invalid assignments (e.g. array → string, array → int|string) with Zend-style TypeError messages matching zend_verify_property_type / zend_type.c.
• Persist union type metadata on property prototypes (unionTypeConstraints, declaredTypeLabel) and handle scalar weak coercion without array/object → string conversion.
• Fix instance readonly property enforcement: php-cfg previously stripped MODIFIER_READONLY from property visibility; new php-cfg-property-readonly.patch preserves the flag (promoted readonly already worked via promotionReadonly).

php-src reference

• Zend/zend_execute.c — ZEND_ASSIGN_OBJ, typed property checks
• Zend/zend_type.c — union assignability
• Zend/zend_object_handlers.c — readonly property guard

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/compliance/VMTest.php --filter typed_property'
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/compliance/VMTest.php --filter promoted_readonly'

Repro (VM now matches Zend):

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php -r "
class C { public string \$x = \"\"; \$c = new C(); try { \$c->x = []; } catch (Throwable \$e) { echo \$e->getMessage(); }
"'
# Cannot assign array to property C::$x of type string

Closes #169

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