Function run_to_outcome 

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

Compile and run, preserving partial output alongside any error — the shape the differential harness compares against the tree-walker (which also keeps the lines emitted before a runtime error). types carries the discovery pass’s struct definitions (for default-fill on construction).