Forward class keyword arguments in mypyc - #1

Fixes python#21938

Class-definition keyword arguments were preserved in mypy's AST but silently dropped by mypyc. This change:

• forwards keywords to init_subclass for native extension classes
• forwards keywords to the metaclass constructor for non-native classes
• excludes metaclass= because Python consumes it for metaclass selection
• evaluates keyword expressions before prepare and the class body

Tests:

• added native and non-native runtime regression coverage
• updated IR expectations for the expanded runtime operation
• mypyc class runtime suite: 125 passed
• mypyc class IR suite: 64 passed
• mypyc self-check: passed
• pre-commit hooks: passed

I used an AI coding assistant to help investigate the issue and draft the implementation; I reviewed the resulting code and test behavior locally.