fix: Kilo/CHIP-8アプリエラー修正 - toInteger, substring, レガシーBox削除

## 修正内容
1. **toIntegerメソッド実装** (#125)
   - StringBoxにtoInteger()メソッド追加
   - box_trait::IntegerBoxを返すよう統一（レガシーboxes::IntegerBox削除）

2. **substringメソッド実装**
   - StringBoxにsubstring(start, end)メソッド追加
   - Kiloエディタで必要な文字列操作を完全サポート

3. **レガシーコード削除**
   - src/boxes/mod.rsから重複StringBox/IntegerBox/BoolBoxエクスポート削除
   - 全てbox_trait実装に統一

4. **プラグインドキュメント整理**
   - 古い仕様書に「理想案・未実装」「将来構想」明記
   - 実装ベースの正確な仕様書作成
   - migration-guide.md追加

## テスト結果
- ✅ Kiloエディタ: 完全動作確認（"Enhanced Kilo Editor test complete"）
- ✅ toInteger()の乗算: 正常動作
- ✅ substring(): 正常動作

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/boxes/string_box.rs

@@ -68,7 +68,7 @@ impl StringBox {
     
     /// Find substring and return position (or -1 if not found)
     pub fn find(&self, search: &str) -> Box<dyn NyashBox> {
-        use crate::boxes::IntegerBox;
+        use crate::boxes::integer_box::IntegerBox;
         match self.value.find(search) {
             Some(pos) => Box::new(IntegerBox::new(pos as i64)),
             None => Box::new(IntegerBox::new(-1)),
@@ -97,19 +97,19 @@ impl StringBox {
     
     /// Check if string contains substring
     pub fn contains(&self, search: &str) -> Box<dyn NyashBox> {
-        use crate::boxes::BoolBox;
+        use crate::boxes::bool_box::BoolBox;
         Box::new(BoolBox::new(self.value.contains(search)))
     }
     
     /// Check if string starts with prefix
     pub fn starts_with(&self, prefix: &str) -> Box<dyn NyashBox> {
-        use crate::boxes::BoolBox;
+        use crate::boxes::bool_box::BoolBox;
         Box::new(BoolBox::new(self.value.starts_with(prefix)))
     }
     
     /// Check if string ends with suffix
     pub fn ends_with(&self, suffix: &str) -> Box<dyn NyashBox> {
-        use crate::boxes::BoolBox;
+        use crate::boxes::bool_box::BoolBox;
         Box::new(BoolBox::new(self.value.ends_with(suffix)))
     }
     
@@ -127,6 +127,18 @@ impl StringBox {
             Box::new(StringBox::new(array_box.to_string_box().value))
         }
     }
+    
+    /// Convert string to integer (parse as i64)
+    pub fn to_integer(&self) -> Box<dyn NyashBox> {
+        use crate::boxes::integer_box::IntegerBox;
+        match self.value.trim().parse::<i64>() {
+            Ok(n) => Box::new(IntegerBox::new(n)),
+            Err(_) => {
+                // If parsing fails, return 0 (JavaScript-like behavior)
+                Box::new(IntegerBox::new(0))
+            }
+        }
+    }
 }
 
 impl NyashBox for StringBox {