Phase 30.1 & 73: Stage-3 features env and JoinIR flag cleanup

src/mir/builder/if_form.rs

@@ -213,6 +213,8 @@ impl MirBuilder {
         let pre_then_var_value: Option<ValueId> = None;
 
         // Phase 61-4: JoinIR 経路試行（ループ外 If）
+        //
+        // - 実際の有効化判定は config::env 側のポリシー関数に集約する。
         let joinir_enabled = crate::config::env::joinir_if_select_enabled();
         let joinir_toplevel = crate::config::env::joinir_if_toplevel_enabled();
         let joinir_dryrun = crate::config::env::joinir_if_toplevel_dryrun_enabled();