Function decision_fabric_from_source 

pub fn decision_fabric_from_source(src: &str, fn_name: &str) -> DecisionFabric

The Futamura P1 lowering for a branchy decision. Compile src, take function fn_name’s straight-line-with-branches body (entry .. Return), and if-convert it to a combinational select-DAG: parameters become circuit inputs, every If a Select. Panics on a shape outside the compare/arith/branch family (the honest boundary — loops map to a CGRA, division to a separate functional unit).