feat(control_tree): Phase 123 P1 return integer literal in Normalized if-only

- Add value_ast to StepStmtKind::Return for payload tracking
- Implement lower_return_value for Integer literal → Compute(Const) + Ret
- Add 3 unit tests: integer literal, void, variable fail-fast
- All tests passing (7 passed)

src/mir/control_tree/normalized_shadow/builder.rs

@@ -61,7 +61,7 @@ impl StepTreeNormalizedShadowLowererBox {
         }
     }
 
-    /// Lower if-only StepTree to Normalized JoinModule (Phase 122)
+    /// Lower if-only StepTree to Normalized JoinModule (Phase 122-123)
     ///
     /// ## Design
     ///
@@ -69,6 +69,12 @@ impl StepTreeNormalizedShadowLowererBox {
     /// - 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)
     ///
+    /// ## Phase 123 Node Support
+    ///
+    /// - Return(Integer literal): `Const + Ret(Some(vid))`
+    /// - Return(Variable): Fail-Fast (needs reads fact)
+    /// - Return(void): `Ret(None)`
+    ///
     /// ## Returns
     ///
     /// - `Ok((module, meta))`: Normalized JoinModule生成成功
@@ -80,11 +86,8 @@ impl StepTreeNormalizedShadowLowererBox {
         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
+        // Phase 122: env レイアウト
         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
@@ -98,16 +101,19 @@ impl StepTreeNormalizedShadowLowererBox {
             })
             .collect();
 
-        // main 関数生成（最小: Ret のみ）
+        // main 関数生成
         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 });
+        // Phase 123: Return node lowering
+        Self::lower_return_from_tree(
+            &step_tree.root,
+            &mut main_func.body,
+            &mut next_value_id,
+        )?;
 
         // JoinModule 構築
         let mut module = JoinModule::new();
@@ -121,6 +127,120 @@ impl StepTreeNormalizedShadowLowererBox {
         Ok((module, meta))
     }
 
+    /// Phase 123 P1: Lower Return node from StepTree
+    ///
+    /// ## Support (Phase 123)
+    ///
+    /// - Return(Integer literal): Generate Const + Ret(Some(vid))
+    /// - Return(void): Ret(None)
+    /// - Return(other): Fail-Fast with structured error
+    fn lower_return_from_tree(
+        node: &crate::mir::control_tree::step_tree::StepNode,
+        body: &mut Vec<crate::mir::join_ir::JoinInst>,
+        next_value_id: &mut u32,
+    ) -> Result<(), String> {
+        use crate::ast::{ASTNode, LiteralValue};
+        use crate::mir::control_tree::step_tree::{StepNode, StepStmtKind};
+        use crate::mir::join_ir::JoinInst;
+        use crate::mir::ValueId;
+
+        match node {
+            StepNode::Block(nodes) => {
+                // Find first Return in block (Phase 123 minimal: single return only)
+                for n in nodes {
+                    if let StepNode::Stmt {
+                        kind: StepStmtKind::Return { value_ast },
+                        ..
+                    } = n
+                    {
+                        return Self::lower_return_value(value_ast, body, next_value_id);
+                    }
+                }
+                // No return found - default to void
+                body.push(JoinInst::Ret { value: None });
+                Ok(())
+            }
+            StepNode::Stmt {
+                kind: StepStmtKind::Return { value_ast },
+                ..
+            } => Self::lower_return_value(value_ast, body, next_value_id),
+            _ => {
+                // No return in tree - default to void
+                body.push(JoinInst::Ret { value: None });
+                Ok(())
+            }
+        }
+    }
+
+    /// Phase 123 P1-P2: Lower return value
+    ///
+    /// ## Support
+    ///
+    /// - Integer literal: Generate Const + Ret(Some(vid))
+    /// - None: Ret(None)
+    /// - Variable: Fail-Fast (needs reads fact - Phase 124)
+    /// - Other: Fail-Fast (out of scope)
+    fn lower_return_value(
+        value_ast: &Option<crate::mir::control_tree::step_tree::AstNodeHandle>,
+        body: &mut Vec<crate::mir::join_ir::JoinInst>,
+        next_value_id: &mut u32,
+    ) -> Result<(), String> {
+        use crate::ast::{ASTNode, LiteralValue};
+        use crate::mir::join_ir::{ConstValue, JoinInst, MirLikeInst};
+        use crate::mir::ValueId;
+
+        match value_ast {
+            None => {
+                body.push(JoinInst::Ret { value: None });
+                Ok(())
+            }
+            Some(ast_handle) => {
+                let ast = &ast_handle.0;
+                match &**ast {
+                    ASTNode::Literal { value, .. } => match value {
+                        LiteralValue::Integer(i) => {
+                            // Phase 123 P1: Integer literal → Const + Ret(Some(vid))
+                            let const_vid = ValueId(*next_value_id);
+                            *next_value_id += 1;
+
+                            // Generate Const instruction (wrapped in Compute)
+                            body.push(JoinInst::Compute(MirLikeInst::Const {
+                                dst: const_vid,
+                                value: ConstValue::Integer(*i),
+                            }));
+
+                            // Generate Ret instruction
+                            body.push(JoinInst::Ret {
+                                value: Some(const_vid),
+                            });
+
+                            Ok(())
+                        }
+                        _ => {
+                            // Phase 123: Other literals not supported
+                            Err(format!(
+                                "[phase123/return/literal_unsupported] Phase 123 only supports integer literals. Hint: Use integer literal or wait for Phase 124"
+                            ))
+                        }
+                    },
+                    ASTNode::Variable { name, .. } => {
+                        // Phase 123 P2: Variable not supported (needs reads fact)
+                        Err(format!(
+                            "[phase123/return/var_unsupported] Phase 123 only supports return with integer literals (found variable: {}). Hint: Add reads fact (Phase 124) or return literal only",
+                            name
+                        ))
+                    }
+                    _ => {
+                        // Phase 123: Other expressions not supported
+                        Err(format!(
+                            "[phase123/return/expr_unsupported] Phase 123 only supports integer literals. Hint: Simplify to literal or wait for Phase 124"
+                        ))
+                    }
+                }
+            }
+        }
+    }
+
     /// Get shadow lowering status string for dev logging
     ///
     /// ## Contract
@@ -152,7 +272,9 @@ impl StepTreeNormalizedShadowLowererBox {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::mir::control_tree::step_tree::{StepNode, StepTreeFeatures, StepTreeSignature};
+    use crate::mir::control_tree::step_tree::{
+        StepNode, StepStmtKind, StepTreeFeatures, StepTreeSignature,
+    };
     use crate::mir::control_tree::step_tree_contract_box::StepTreeContract;
 
     fn make_if_only_tree() -> StepTree {
@@ -230,4 +352,115 @@ mod tests {
         assert!(status.contains("shadow_lowered=false"));
         assert!(status.contains("reason=\"contains loop"));
     }
+
+    #[test]
+    fn test_return_integer_literal() {
+        use crate::ast::{ASTNode, LiteralValue, Span};
+        use crate::mir::control_tree::step_tree::AstNodeHandle;
+
+        // Create StepTree with "return 7"
+        let return_ast = Box::new(ASTNode::Literal {
+            value: LiteralValue::Integer(7),
+            span: Span::unknown(),
+        });
+
+        let mut tree = make_if_only_tree();
+        tree.root = StepNode::Stmt {
+            kind: StepStmtKind::Return {
+                value_ast: Some(AstNodeHandle(return_ast)),
+            },
+            span: Span::unknown(),
+        };
+
+        // Lower to JoinModule
+        let result = StepTreeNormalizedShadowLowererBox::try_lower_if_only(&tree);
+        assert!(result.is_ok());
+
+        let (module, _meta) = result.unwrap().expect("Should generate JoinModule");
+
+        // Verify Const + Ret instructions
+        assert_eq!(module.functions.len(), 1);
+        let func = &module.functions.values().next().unwrap();
+        assert_eq!(func.body.len(), 2, "Should have Const + Ret");
+
+        // Check Const instruction
+        use crate::mir::join_ir::{ConstValue, JoinInst, MirLikeInst};
+        use crate::mir::ValueId;
+        if let JoinInst::Compute(MirLikeInst::Const { dst, value }) = &func.body[0] {
+            assert_eq!(*dst, ValueId(1));
+            if let ConstValue::Integer(i) = value {
+                assert_eq!(*i, 7);
+            } else {
+                panic!("Expected Integer const");
+            }
+        } else {
+            panic!("Expected Const instruction");
+        }
+
+        // Check Ret instruction
+        if let JoinInst::Ret { value } = &func.body[1] {
+            assert_eq!(value, &Some(ValueId(1)));
+        } else {
+            panic!("Expected Ret instruction");
+        }
+    }
+
+    #[test]
+    fn test_return_void() {
+        use crate::ast::Span;
+
+        // Create StepTree with "return" (no value)
+        let mut tree = make_if_only_tree();
+        tree.root = StepNode::Stmt {
+            kind: StepStmtKind::Return { value_ast: None },
+            span: Span::unknown(),
+        };
+
+        // Lower to JoinModule
+        let result = StepTreeNormalizedShadowLowererBox::try_lower_if_only(&tree);
+        assert!(result.is_ok());
+
+        let (module, _meta) = result.unwrap().expect("Should generate JoinModule");
+
+        // Verify Ret(None) instruction
+        assert_eq!(module.functions.len(), 1);
+        let func = &module.functions.values().next().unwrap();
+        assert_eq!(func.body.len(), 1, "Should have only Ret");
+
+        // Check Ret instruction
+        use crate::mir::join_ir::JoinInst;
+        if let JoinInst::Ret { value } = &func.body[0] {
+            assert_eq!(value, &None);
+        } else {
+            panic!("Expected Ret instruction");
+        }
+    }
+
+    #[test]
+    fn test_return_variable_fails() {
+        use crate::ast::{ASTNode, Span};
+        use crate::mir::control_tree::step_tree::AstNodeHandle;
+
+        // Create StepTree with "return x" (variable)
+        let return_ast = Box::new(ASTNode::Variable {
+            name: "x".to_string(),
+            span: Span::unknown(),
+        });
+
+        let mut tree = make_if_only_tree();
+        tree.root = StepNode::Stmt {
+            kind: StepStmtKind::Return {
+                value_ast: Some(AstNodeHandle(return_ast)),
+            },
+            span: Span::unknown(),
+        };
+
+        // Lower to JoinModule - should fail
+        let result = StepTreeNormalizedShadowLowererBox::try_lower_if_only(&tree);
+        assert!(result.is_err());
+
+        let err = result.unwrap_err();
+        assert!(err.contains("phase123/return/var_unsupported"));
+        assert!(err.contains("Phase 124"));
+    }
 }