Function run_to_outcome_bg 

pub fn run_to_outcome_bg(
    stmts: &[Stmt<'_>],
    interner: &Interner,
    types: Option<&TypeRegistry>,
    policies: Option<&PolicyRegistry>,
    program_args: &[String],
) -> (String, Option<String>)

run_to_outcome_with_args with BACKGROUND native compilation (HOTSWAP §6): hot functions compile on a worker thread while the interpreter keeps running bytecode, then their chains are drained + published. Requires a process-installed tier (the worker needs a &'static backend); with none installed it runs pure bytecode. Drains outstanding compiles before returning so the native tier engages deterministically for the differential gates.