Function is_read_only_statement 

pub fn is_read_only_statement(stmt: &Statement) -> bool

Mission infra-1: classify a parsed statement as read-only vs. mutating. Used by Engine::execute_powql_readonly and by the server handler to decide between the RwLock reader and writer sides. Union recurses because each side can independently be read/write (though in practice both sides are reads — the parser only builds Union from query shapes).