Language: reject public+private(set) asymmetric visibility at compile time (#7099)

[PurHur]

Summary

• Add rejectExplicitReadBeforeAsymmetricSetModifier() in AsymmetricVisibilityRewriter so explicit get visibility combined with (set) (e.g. public private(set), public (private(set))) fails at compile time with Zend's Multiple access type modifiers are not allowed message.
• Update compliance/unit tests: runtime parity cases now use standalone private(set); invalid double-modifier forms expect compile fatal.

php-src reference: Zend/zend_compile.c — zend_add_member_modifier()

Test plan

• Issue repro — compile fatal before user code runs
• Valid private(set) / promoted private(set) still compile and enforce set visibility at runtime
• PHPUnit asymmetric visibility suite green

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php -r "class A { public private(set) string \$x = \"a\"; }" 2>&1; echo vm_exit=\$?
vendor/bin/phpunit --filter asymmetric_visibility_invalid_modifiers
vendor/bin/phpunit --filter asymmetric_visibility
'

Output (2026-06-06):

parseAndCompile failure: Multiple access type modifiers are not allowed
vm_exit=255
OK (1 test, 1 assertion)
OK (21 tests, 23 assertions)

Closes #7099