feat: Phase 15.5-A 予約型保護解除完了＋StringBox問題調査

🎯 **Phase 15.5 Phase A実装完了**
- ✅ is_reserved_type()修正: 環境変数による条件付きバイパス実装
- ✅ NYASH_USE_PLUGIN_BUILTINS=1 + NYASH_PLUGIN_OVERRIDE_TYPES対応
- ✅ ビルド確認: エラーなしでコンパイル成功
- ✅ プラグインロード確認: 予約型拒否エラーなし

🔍 **StringBox問題完全調査**
- 問題特定: get()/set()が空文字を返す（length=4で内部データあり）
- 全バックエンド影響: VM/プラグイン/コア すべて同じ問題
- MIRレベル正常: newbox/boxcall正しく生成
- 調査資料: /tmp/stringbox_test_results.md完備

📂 **変更ファイル**
- src/box_factory/mod.rs: 予約型保護条件付き解除
- docs/phase-15.5: Phase A完了ドキュメント更新
- nyash.toml: StringBox/IntegerBoxプラグイン設定追加

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

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

nyash.toml

@@ -333,3 +333,45 @@ singleton = false
 birth = { method_id = 0 }
 compile = { method_id = 1 }
 fini = { method_id = 4294967295 }
+# StringBox Plugin (Core Box replacement)
+[libraries."libnyash_string_plugin.so"]
+boxes = ["StringBox"]
+path = "target/release/libnyash_string_plugin.so"
+
+[libraries."libnyash_string_plugin.so".StringBox]
+type_id = 10
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_string_plugin.so".StringBox.methods]
+birth = { method_id = 0 }
+get = { method_id = 1 }
+set = { method_id = 2 }
+concat = { method_id = 3 }
+length = { method_id = 4 }
+substring = { method_id = 5 }
+charCodeAt = { method_id = 6 }
+indexOf = { method_id = 7 }
+lastIndexOf = { method_id = 8 }
+replace = { method_id = 9 }
+split = { method_id = 10 }
+trim = { method_id = 11 }
+toUpper = { method_id = 12 }
+toLower = { method_id = 13 }
+fini = { method_id = 4294967295 }
+
+# IntegerBox Plugin (Core Box replacement)
+[libraries."libnyash_integer_plugin.so"]
+boxes = ["IntegerBox"]
+path = "target/release/libnyash_integer_plugin.so"
+
+[libraries."libnyash_integer_plugin.so".IntegerBox]
+type_id = 12
+abi_version = 1
+singleton = false
+
+[libraries."libnyash_integer_plugin.so".IntegerBox.methods]
+birth = { method_id = 0 }
+get = { method_id = 1 }
+set = { method_id = 2 }
+fini = { method_id = 4294967295 }