✨ feat: ArrayBox完全実装 - Nyash初の動的配列コレクション！

## 🎯 実装内容
- ArrayBoxにNyashBoxトレイト完全実装
- Arc<Mutex>による内部可変性実現（MapBoxと同じパターン）
- 全メソッド実装: push/pop/get/set/length/indexOf/contains/join/clear/remove
- 包括的なテストスイート作成

## 🔧 技術的改善
- GitHub Copilot作成の基本構造をNyash対応に拡張
- execute_array_methodを&self参照に修正
- collection_methods.rsとの統合完了

## ✅ テスト結果
- 全機能正常動作確認
- 異なる型の要素混在可能（Everything is Box哲学）
- インデックス範囲外アクセスでNullBox返却

## 📝 残課題
- BufferBox, ResultBox, FileBox等は基本構造のみ（未実装）
- RegexBoxはNyashBoxトレイト実装済みだがregex依存関係未追加

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

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

src/interpreter/methods/collection_methods.rs

@@ -8,7 +8,8 @@
  */
 
 use super::super::*;
-use crate::box_trait::{StringBox, IntegerBox, ArrayBox, NyashBox, VoidBox, BoolBox};
+use crate::box_trait::{StringBox, IntegerBox, NyashBox, BoolBox};
+use crate::boxes::array::ArrayBox;
 use crate::boxes::map_box::MapBox;
 
 impl NyashInterpreter {
@@ -48,17 +49,7 @@ impl NyashInterpreter {
                     });
                 }
                 let index_value = self.execute_expression(&arguments[0])?;
-                if let Some(index_int) = index_value.as_any().downcast_ref::<IntegerBox>() {
-                    if let Some(element) = array_box.get(index_int.value as usize) {
-                        Ok(element)
-                    } else {
-                        Ok(Box::new(StringBox::new("Index out of bounds")))
-                    }
-                } else {
-                    Err(RuntimeError::TypeError {
-                        message: "get() requires integer index".to_string(),
-                    })
-                }
+                Ok(array_box.get(index_value))
             }
             "set" => {
                 if arguments.len() != 2 {
@@ -68,18 +59,43 @@ impl NyashInterpreter {
                 }
                 let index_value = self.execute_expression(&arguments[0])?;
                 let element_value = self.execute_expression(&arguments[1])?;
-                if let Some(index_int) = index_value.as_any().downcast_ref::<IntegerBox>() {
-                    match array_box.set(index_int.value as usize, element_value) {
-                        Ok(()) => Ok(Box::new(VoidBox::new())),
-                        Err(msg) => Ok(Box::new(StringBox::new(&msg))),
-                    }
-                } else {
-                    Err(RuntimeError::TypeError {
-                        message: "set() requires integer index".to_string(),
-                    })
-                }
+                Ok(array_box.set(index_value, element_value))
+            }
+            "remove" => {
+                if arguments.len() != 1 {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("remove() expects 1 argument, got {}", arguments.len()),
+                    });
+                }
+                let index_value = self.execute_expression(&arguments[0])?;
+                Ok(array_box.remove(index_value))
+            }
+            "indexOf" => {
+                if arguments.len() != 1 {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("indexOf() expects 1 argument, got {}", arguments.len()),
+                    });
+                }
+                let element = self.execute_expression(&arguments[0])?;
+                Ok(array_box.indexOf(element))
+            }
+            "contains" => {
+                if arguments.len() != 1 {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("contains() expects 1 argument, got {}", arguments.len()),
+                    });
+                }
+                let element = self.execute_expression(&arguments[0])?;
+                Ok(array_box.contains(element))
+            }
+            "clear" => {
+                if !arguments.is_empty() {
+                    return Err(RuntimeError::InvalidOperation {
+                        message: format!("clear() expects 0 arguments, got {}", arguments.len()),
+                    });
+                }
+                Ok(array_box.clear())
             }
-            // Note: indexOf, contains, clear, reverse, slice methods not implemented in ArrayBox yet
             "join" => {
                 if arguments.len() != 1 {
                     return Err(RuntimeError::InvalidOperation {
@@ -87,13 +103,7 @@ impl NyashInterpreter {
                     });
                 }
                 let delimiter_value = self.execute_expression(&arguments[0])?;
-                if let Some(delimiter_str) = delimiter_value.as_any().downcast_ref::<StringBox>() {
-                    Ok(array_box.join(&delimiter_str.value))
-                } else {
-                    Err(RuntimeError::TypeError {
-                        message: "join() requires string delimiter".to_string(),
-                    })
-                }
+                Ok(array_box.join(delimiter_value))
             }
             "isEmpty" => {
                 if !arguments.is_empty() {