Stdlib: ReflectionFunction::isVariadic() returns Attribute-style flag (#22045)

[PurHur]

Summary

• Implement ReflectionFunction::isVariadic() (VM + JIT) matching php-src zim_reflection_function_abstract_isVariadic for free functions, closures, and known variadic builtins.
• Sibling of Stdlib: ReflectionMethod::isInternal()/isVariadic() missing (ext/reflection/php_reflection.c) #18228 (ReflectionMethod::isVariadic); PHP-in-PHP only — no new C runtime.
• JIT/AOT Call + construct/getName proxies + compile-time variadic lookup table for standalone lowering.

php-src reference: ext/reflection/php_reflection.c — zim_reflection_function_abstract_isVariadic

Closes #22045

Test plan

• Zend vs VM: test/repro/maintainer_gap_reflection_function_is_variadic.php
• JIT: php bin/jit.php test/repro/maintainer_gap_reflection_function_is_variadic.php
• Compliance: reflection_function_is_variadic.phpt (VM+JIT via phpunit)
• Inventory 6341/6341 + spine coverage + sidecar stamp sync

Verification transcript

$ php bin/vm.php test/repro/maintainer_gap_reflection_function_is_variadic.php
f_variadic=true
g_variadic=false
f_params=3
f_required=1
closure_variadic=true
method_variadic=true
call_user_func=true
strlen=false

$ php bin/jit.php test/repro/maintainer_gap_reflection_function_is_variadic.php
(same)

$ php vendor/bin/phpunit --filter reflection_function_is_variadic
OK (2 tests, 2 assertions)

$ php script/bootstrap-inventory.php --check
OK 6341/6341