Span trace utilities and runner source hint

src/mir/loop_form.rs

@@ -0,0 +1,6 @@
+//! LoopForm helper alias
+//!
+//! Alias to ControlForm::LoopShape so generic JoinIR lowering can depend on a
+//! stable loop shape representation without pulling in builder details.
+
+pub type LoopForm = crate::mir::control_form::LoopShape;