feat(joinir): S-5.2-improved - VM完全意味論統合完了

Phase 27-shortterm S-5.2-improved 実装完了:

## 実装内容
1. **execute_box_call() ラッパー追加**:
   - src/backend/mir_interpreter/mod.rs に公開 API 実装
   - 1_000_000 番台レジスタで一時値管理
   - 適切なクリーンアップ処理実装

2. **handle_box_call 可視性変更**:
   - src/backend/mir_interpreter/handlers/boxes.rs を pub に変更
   - JoinIR Runner からアクセス可能に

3. **JoinIR Runner BoxCall 統合**:
   - src/mir/join_ir_runner.rs の BoxCall 処理を書き換え
   - StringBox 直接呼び出し削除
   - VM の execute_box_call 経由に変更
   - JoinValue ↔ VMValue 変換で統合

4. **不要な関数削除**:
   - expect_str(), expect_int(), box_to_join_value() 削除
   - VMValue 変換に一本化

5. **テスト更新**:
   - joinir_runner_standalone.rs: VM インスタンス追加
   - joinir_runner_min.rs: VM 再利用

## 期待される効果
✅ Void guards 完全対応 (Void.length() → 0)
✅ PluginBox/InstanceBox 将来対応可能
✅ VM の完全な BoxCall 意味論統一
✅ VM 2号機回避（ガードレール設計成功）

## テスト結果
✅ joinir_runner_standalone_skip_ws ... ok
✅ joinir_runner_standalone_trim ... ok
✅ ビルド成功（0エラー）

## 完了タスク
- [x] MirInterpreter::execute_box_call() 実装
- [x] 1_000_000 レジスタ帯域割り当て
- [x] regs クリーンアップ実装
- [x] JoinIR Runner BoxCall 書き換え
- [x] テスト更新＆PASS確認

🎉 VM 完全意味論統合完了！

src/tests/joinir_runner_min.rs

@@ -65,7 +65,9 @@ static box Runner {
 
     let join_module =
         lower_skip_ws_to_joinir(&compiled.module).expect("lower_skip_ws_to_joinir failed");
+    // S-5.2-improved: Reuse VM instance for JoinIR Runner
     let join_result = run_joinir_function(
+        &mut vm,
         &join_module,
         JoinFuncId::new(0),
         &[JoinValue::Str("   abc".to_string())],
@@ -126,7 +128,9 @@ static box Runner {
 
     let join_module = lower_funcscanner_trim_to_joinir(&compiled.module)
         .expect("lower_funcscanner_trim_to_joinir failed");
+    // S-5.2-improved: Reuse VM instance for JoinIR Runner
     let join_result = run_joinir_function(
+        &mut vm,
         &join_module,
         JoinFuncId::new(0),
         &[JoinValue::Str("   abc  ".to_string())],

src/tests/joinir_runner_standalone.rs

@@ -60,8 +60,12 @@ fn joinir_runner_standalone_skip_ws() {
 
     let join_module = build_skip_ws_joinir();
 
+    // S-5.2-improved: Create MirInterpreter instance for VM integration
+    let mut vm = crate::backend::mir_interpreter::MirInterpreter::new();
+
     // Test case 1: "   abc" → 3
     let result = run_joinir_function(
+        &mut vm,
         &join_module,
         JoinFuncId::new(0), // entry function
         &[JoinValue::Str("   abc".to_string())],
@@ -74,6 +78,7 @@ fn joinir_runner_standalone_skip_ws() {
 
     // Test case 2: "" → 0
     let result_empty = run_joinir_function(
+        &mut vm,
         &join_module,
         JoinFuncId::new(0),
         &[JoinValue::Str("".to_string())],
@@ -86,6 +91,7 @@ fn joinir_runner_standalone_skip_ws() {
 
     // Test case 3: "abc" → 0
     let result_no_ws = run_joinir_function(
+        &mut vm,
         &join_module,
         JoinFuncId::new(0),
         &[JoinValue::Str("abc".to_string())],
@@ -143,8 +149,12 @@ fn joinir_runner_standalone_trim() {
 
     let join_module = build_trim_joinir();
 
+    // S-5.2-improved: Create MirInterpreter instance for VM integration
+    let mut vm = crate::backend::mir_interpreter::MirInterpreter::new();
+
     // Test case 1: "   abc  " → "abc  " (simplified - only leading whitespace)
     let result = run_joinir_function(
+        &mut vm,
         &join_module,
         JoinFuncId::new(0), // entry function
         &[JoinValue::Str("   abc  ".to_string())],
@@ -157,6 +167,7 @@ fn joinir_runner_standalone_trim() {
 
     // Test case 2: "" → ""
     let result_empty = run_joinir_function(
+        &mut vm,
         &join_module,
         JoinFuncId::new(0),
         &[JoinValue::Str("".to_string())],
@@ -169,6 +180,7 @@ fn joinir_runner_standalone_trim() {
 
     // Test case 3: "abc" → "abc"
     let result_no_ws = run_joinir_function(
+        &mut vm,
         &join_module,
         JoinFuncId::new(0),
         &[JoinValue::Str("abc".to_string())],