feat(control_tree): add StepTree builder (dev-only)

src/mir/mod.rs

@@ -27,6 +27,7 @@ pub mod utils; // Phase 15 control flow utilities for root treatment
                // pub mod lowerers; // reserved: Stage-3 loop lowering (while/for-range)
 pub mod cfg_extractor; // Phase 154: CFG extraction for hako_check
 pub mod control_form;
+pub mod control_tree; // Phase 110: Structure-only SSOT (StepTree)
 pub mod function_emission; // FunctionEmissionBox（MirFunction直編集の発行ヘルパ）
 pub mod hints; // scaffold: zero-cost guidance (no-op)
 pub mod join_ir; // Phase 26-H: 関数正規化IR（JoinIR）