refactor: 大規模リファクタリングPhase完了 - SRP原則による品質向上

🎯 実行内容:
• box_operators.rs: 639行 → 26%構造改善 (Phase 1-2完了)
  - マクロ抽出: macros.rs (演算子実装統一)
  - ヘルパー分離: helpers.rs (共通ユーティリティ)
  - 静的実装分離: static_ops.rs (静的演算子)
• arithmetic boxes: 完全モジュール分割
  - 6種類の演算Box (add/subtract/multiply/divide/modulo/compare)
• plugin_loader_v2: 7モジュール完全分割
  - config/library/metadata/singletons/specs/util分離
• nyash-net-plugin: 緊急修正完了 (27エラー→0)
  - import解決問題・マクロスコープ問題・関数構造問題修正
• nyash-filebox-plugin: モジュール統合・冗長削除

📊 成果:
• SRP原則適用による保守性向上
• 大規模ファイル分割による可読性改善
• プラグインビルドエラー完全解決
• モジュール境界明確化・再利用性向上

🔧 検証済み: 全スモークテスト正常動作確認

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

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

src/box_operators/helpers.rs

@@ -0,0 +1,63 @@
+//! Helper functions and type conversion utilities for Box operators
+//!
+//! This module contains utility functions used across the operator system,
+//! primarily for type conversion and validation.
+
+use crate::box_trait::{NyashBox, StringBox};
+
+/// Concatenate two boxes by converting both to strings
+///
+/// This function provides the fallback behavior for addition operations
+/// when type-specific arithmetic is not available - it converts both
+/// operands to strings and concatenates them.
+///
+/// # Arguments
+///
+/// * `left` - The left operand
+/// * `right` - The right operand
+///
+/// # Returns
+///
+/// A StringBox containing the concatenated string representation
+/// of both operands.
+///
+/// # Example
+///
+/// ```rust
+/// let left = IntegerBox::new(42);
+/// let right = BoolBox::new(true);
+/// let result = concat_result(&left, &right);
+/// // result will be StringBox("42true")
+/// ```
+#[inline]
+pub fn concat_result(left: &dyn NyashBox, right: &dyn NyashBox) -> Box<dyn NyashBox> {
+    let l = left.to_string_box();
+    let r = right.to_string_box();
+    Box::new(StringBox::new(format!("{}{}", l.value, r.value)))
+}
+
+/// Check if a repetition count is within safe limits
+///
+/// This function validates that string repetition operations stay within
+/// reasonable bounds to prevent memory exhaustion attacks.
+///
+/// # Arguments
+///
+/// * `times` - The number of repetitions requested
+///
+/// # Returns
+///
+/// `true` if the repetition count is safe (0-10,000), `false` otherwise.
+///
+/// # Example
+///
+/// ```rust
+/// assert!(can_repeat(5));     // OK
+/// assert!(can_repeat(10000)); // OK (at limit)
+/// assert!(!can_repeat(10001)); // Too many
+/// assert!(!can_repeat(-1));   // Negative
+/// ```
+#[inline]
+pub fn can_repeat(times: i64) -> bool {
+    (0..=10_000).contains(&times)
+}