Function vm_run_source 

pub fn vm_run_source(source: &str) -> Result<String, String>

Parse LOGOS source and execute it on the register bytecode VM, returning the captured output. Uses the SAME front-end as the tree-walker ([crate::ui_bridge::with_parsed_program]: lex → MWE → discovery → parse), so differential tests compare two engines on one program, never two programs. Returns Err for programs outside the VM’s supported subset (the compiler reports vm: unsupported …).