feat(control_tree): emit Normalized JoinModule for if-only (dev-only)

Phase 122 P1: StepTree→Normalized JoinModule generation
- Generate env layout from writes (SSOT, deterministic)
- Minimal implementation: main function + Ret only
- Full If/Assign/Return lowering in future P2-P4
- Dev-only: no behavior change to existing path

src/mir/control_tree/normalized_shadow/builder.rs

@@ -36,6 +36,13 @@ impl StepTreeNormalizedShadowLowererBox {
     /// - Only processes if-only patterns (no loops/breaks/continues)
     /// - Uses contract information only (no AST re-analysis)
     /// - Dev-only: caller must check `joinir_dev_enabled()` before calling
+    ///
+    /// ## Phase 122 Implementation
+    ///
+    /// - Generates Normalized JoinIR (env + continuation)
+    /// - env layout: writes only (SSOT)
+    /// - merge = join_k(env) tail-call (no PHI)
+    /// - Minimal node support: If/Return/Assign(Const/Variable/BinOp(Add))
     pub fn try_lower_if_only(
         step_tree: &StepTree,
     ) -> Result<Option<(JoinModule, JoinFragmentMeta)>, String> {
@@ -43,19 +50,77 @@ impl StepTreeNormalizedShadowLowererBox {
         let capability = check_if_only(step_tree);
         match capability {
             CapabilityCheckResult::Supported => {
-                // Phase 121 P1: For now, return empty module + empty meta
-                // Full lowering implementation in future phases
-                let module = JoinModule::new();
-                let meta = JoinFragmentMeta::empty();
-                Ok(Some((module, meta)))
+                // Phase 122 P1: Generate Normalized JoinModule
+                Self::lower_if_only_to_normalized(step_tree)
+                    .map(Some)
             }
             CapabilityCheckResult::Unsupported(_reason) => {
-                // Out of scope for Phase 121
+                // Out of scope for Phase 121/122
                 Ok(None)
             }
         }
     }
 
+    /// Lower if-only StepTree to Normalized JoinModule (Phase 122)
+    ///
+    /// ## Design
+    ///
+    /// - env レイアウト: `writes` に含まれる変数だけ（決定的順序）
+    /// - merge 形式: `join_k(env)` への tail-call（PHI 禁止）
+    /// - 対応ノード: If/Return/Assign(最小セット)
+    ///
+    /// ## Returns
+    ///
+    /// - `Ok((module, meta))`: Normalized JoinModule生成成功
+    /// - `Err(msg)`: 生成できるはずなのに失敗（内部エラー）
+    fn lower_if_only_to_normalized(
+        step_tree: &StepTree,
+    ) -> Result<(JoinModule, JoinFragmentMeta), String> {
+        use crate::mir::join_ir::{JoinFunction, JoinFuncId, JoinInst};
+        use crate::mir::ValueId;
+        use std::collections::BTreeMap;
+
+        // Phase 122 P1: 最小実装 - main関数1つ + Ret のみ
+        // env レイアウト: writes から決定的に決める
+        let env_fields: Vec<String> = step_tree.contract.writes.iter().cloned().collect();
+
+        // 関数ID生成（Phase 122では固定ID使用）
+        let main_func_id = JoinFuncId::new(0);
+
+        // env フィールドに対応する引数ValueIdを生成
+        let mut next_value_id = 1;
+        let env_params: Vec<ValueId> = env_fields
+            .iter()
+            .map(|_| {
+                let vid = ValueId(next_value_id);
+                next_value_id += 1;
+                vid
+            })
+            .collect();
+
+        // main 関数生成（最小: Ret のみ）
+        let mut main_func = JoinFunction::new(
+            main_func_id,
+            "main".to_string(),
+            env_params.clone(),
+        );
+
+        // Phase 122 P1: 最小実装 - Return void のみ
+        // TODO Phase 122 P2-P4: If/Assign/条件式の lowering 実装
+        main_func.body.push(JoinInst::Ret { value: None });
+
+        // JoinModule 構築
+        let mut module = JoinModule::new();
+        module.add_function(main_func);
+        module.entry = Some(main_func_id);
+        module.mark_normalized();
+
+        // JoinFragmentMeta 生成（最小）
+        let meta = JoinFragmentMeta::empty();
+
+        Ok((module, meta))
+    }
+
     /// Get shadow lowering status string for dev logging
     ///
     /// ## Contract