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/backend/mir_interpreter/mod.rs

@@ -79,6 +79,71 @@ impl MirInterpreter {
         self.static_box_decls.insert(name, decl);
     }
 
+    /// Execute a BoxCall with VM's complete semantics (Phase 27-shortterm S-5.2-improved)
+    ///
+    /// This wrapper allows external modules (e.g., JoinIR Runner) to invoke BoxCall
+    /// with the VM's complete semantics including:
+    /// - Void guards (e.g., Void.length() → 0)
+    /// - PluginBox support (FileBox, NetBox, etc.)
+    /// - InstanceBox policy checks
+    /// - object_fields handling
+    /// - Method re-routing (toString→str)
+    ///
+    /// # Implementation Notes
+    /// - Uses 1_000_000 register range for scratch registers (避ける ID 衝突)
+    /// - Properly cleans up temporary registers after use
+    /// - Delegates to `handle_box_call` for complete VM semantics
+    ///
+    /// # Arguments
+    /// - `receiver`: The box instance (VMValue::BoxRef or primitive)
+    /// - `method`: Method name to invoke
+    /// - `args`: Method arguments as VMValue
+    ///
+    /// # Returns
+    /// Result value as VMValue (may be Void, Int, String, BoxRef, etc.)
+    pub fn execute_box_call(
+        &mut self,
+        receiver: VMValue,
+        method: &str,
+        args: Vec<VMValue>,
+    ) -> Result<VMValue, VMError> {
+        // Allocate temporary register IDs in the 1_000_000 range (not 1000!)
+        // This avoids conflicts with user code and future extensions
+        let base = ValueId(1_000_000);
+        let recv_id = base;
+
+        // Place receiver in register
+        self.regs.insert(recv_id, receiver);
+
+        // Place arguments in consecutive registers
+        let arg_ids: Vec<ValueId> = args
+            .into_iter()
+            .enumerate()
+            .map(|(i, v)| {
+                let id = ValueId(base.0 + 1 + i as u32);
+                self.regs.insert(id, v);
+                id
+            })
+            .collect();
+
+        // Allocate destination register
+        let dst_id = ValueId(base.0 + 1000);
+
+        // Invoke handle_box_call for complete VM semantics
+        self.handle_box_call(Some(dst_id), recv_id, method, &arg_ids)?;
+
+        // Read result (may be Void if method returns nothing)
+        let result = self.regs.remove(&dst_id).unwrap_or(VMValue::Void);
+
+        // Cleanup temporary registers (important to avoid stale values!)
+        self.regs.remove(&recv_id);
+        for id in arg_ids {
+            self.regs.remove(&id);
+        }
+
+        Ok(result)
+    }
+
     /// Ensure static box singleton instance exists, create if not
     /// Returns mutable reference to the singleton instance
     fn ensure_static_box_instance(