feat(joinir): Phase 27-shortterm S-5.1 完了 - JoinValue を VMValue ベースに統合

## 実装内容

### JoinValue 型の拡張
- **BoxRef variant 追加**: `BoxRef(Arc<dyn NyashBox>)` で VM と値を共有
- **Manual PartialEq 実装**: BoxRef は Arc::ptr_eq で比較

### VMValue との相互変換強化
- `to_vm_value()`: BoxRef サポート追加
- `into_vm_value()`: Zero-cost 変換（owned values）
- `from_vm_value()`: BoxRef サポート追加

### 後方互換性
- 既存の Int/Bool/Str/Unit variant は変更なし
- すべての既存コードがそのまま動作
- 16個の unit test 全て PASS ✅

### S-5.2 への準備
- BoxRef を method_router 経由で使用する準備完了
- 「VM 2号機」を避けるための基盤確立

## テスト結果
- join_ir_ops unit tests: 16 passed / 0 failed

src/mir/join_ir_ops.rs

@@ -11,12 +11,32 @@
 use crate::mir::join_ir::{BinOpKind, CompareOp};
 
 /// JoinIR で扱う値型
-#[derive(Debug, Clone, PartialEq)]
+///
+/// Phase 27-shortterm S-5.1: VMValue との統合
+/// - BoxRef variant 追加（S-5.2 で method_router 統合時に使用）
+/// - from_vm_value()/into_vm_value() で VMValue と相互変換可能
+#[derive(Debug, Clone)]
 pub enum JoinValue {
     Int(i64),
     Bool(bool),
     Str(String),
     Unit,
+    /// S-5.2: method_router 経由で Box/Plugin を実行するために使用
+    BoxRef(std::sync::Arc<dyn crate::box_trait::NyashBox>),
+}
+
+// Manual PartialEq implementation (BoxRef は Arc::ptr_eq で比較)
+impl PartialEq for JoinValue {
+    fn eq(&self, other: &Self) -> bool {
+        match (self, other) {
+            (JoinValue::Int(a), JoinValue::Int(b)) => a == b,
+            (JoinValue::Bool(a), JoinValue::Bool(b)) => a == b,
+            (JoinValue::Str(a), JoinValue::Str(b)) => a == b,
+            (JoinValue::Unit, JoinValue::Unit) => true,
+            (JoinValue::BoxRef(a), JoinValue::BoxRef(b)) => std::sync::Arc::ptr_eq(a, b),
+            _ => false,
+        }
+    }
 }
 
 /// JoinIR ops box エラー型
@@ -288,6 +308,8 @@ mod tests {
 impl JoinValue {
     /// Convert JoinValue to VMValue for VM execution
     ///
+    /// Phase 27-shortterm S-5.1: BoxRef サポート追加
+    ///
     /// # Example
     /// ```ignore
     /// let join_val = JoinValue::Int(42);
@@ -300,13 +322,28 @@ impl JoinValue {
             JoinValue::Bool(b) => crate::backend::VMValue::Bool(*b),
             JoinValue::Str(s) => crate::backend::VMValue::String(s.clone()),
             JoinValue::Unit => crate::backend::VMValue::Void,
+            JoinValue::BoxRef(b) => crate::backend::VMValue::BoxRef(b.clone()),
+        }
+    }
+
+    /// Convert JoinValue into VMValue (consuming self)
+    ///
+    /// Phase 27-shortterm S-5.1: Zero-cost conversion for owned values
+    pub fn into_vm_value(self) -> crate::backend::VMValue {
+        match self {
+            JoinValue::Int(i) => crate::backend::VMValue::Integer(i),
+            JoinValue::Bool(b) => crate::backend::VMValue::Bool(b),
+            JoinValue::Str(s) => crate::backend::VMValue::String(s),
+            JoinValue::Unit => crate::backend::VMValue::Void,
+            JoinValue::BoxRef(b) => crate::backend::VMValue::BoxRef(b),
         }
     }
 
     /// Convert VMValue to JoinValue
     ///
-    /// Returns error for VMValue types not representable in JoinValue
-    /// (Float, Future, BoxRef).
+    /// Phase 27-shortterm S-5.1: BoxRef サポート追加
+    ///
+    /// Returns error for VMValue types not representable in JoinValue (Float, Future).
     ///
     /// # Example
     /// ```ignore
@@ -320,15 +357,13 @@ impl JoinValue {
             crate::backend::VMValue::Bool(b) => Ok(JoinValue::Bool(*b)),
             crate::backend::VMValue::String(s) => Ok(JoinValue::Str(s.clone())),
             crate::backend::VMValue::Void => Ok(JoinValue::Unit),
+            crate::backend::VMValue::BoxRef(b) => Ok(JoinValue::BoxRef(b.clone())),
             crate::backend::VMValue::Float(_) => {
                 Err(JoinIrOpError::new("Float not supported in JoinValue"))
             }
             crate::backend::VMValue::Future(_) => {
                 Err(JoinIrOpError::new("Future not supported in JoinValue"))
             }
-            crate::backend::VMValue::BoxRef(_) => {
-                Err(JoinIrOpError::new("BoxRef not directly convertible to JoinValue - requires NyashBox downcast"))
-            }
         }
     }
 }