refactor: MIR instruction.rs 4-Phase大型リファクタリング完了（888→315行、64%削減）

Single Responsibility Principle適用による完全分離:
- Phase 1: テスト分離 → instruction/tests.rs (196行)
- Phase 2: Display実装分離 → instruction/display.rs (130行)
- Phase 3: メソッド実装分離 → instruction/methods.rs (247行)
- Phase 4: 統合テスト成功（全コンパイルエラー解決）

技術的成果:
- MirInstruction enumを単一責任に集中
- 各実装が独立して保守可能な構造
- EffectMask::read→READ修正も完了
- ビルド成功確認済み（警告のみ）

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

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

src/boxes/basic/mod.rs

@@ -0,0 +1,20 @@
+//! Basic box implementations
+//!
+//! This module contains the core basic Box types that implement the
+//! fundamental data types in Nyash: String, Integer, Boolean, Void, File, and Error.
+
+// Individual basic box implementations
+mod string_box;
+mod integer_box;
+mod bool_box;
+mod void_box;
+mod file_box;
+mod error_box;
+
+// Re-export all basic box types
+pub use string_box::StringBox;
+pub use integer_box::IntegerBox;
+pub use bool_box::BoolBox;
+pub use void_box::VoidBox;
+pub use file_box::FileBox;
+pub use error_box::ErrorBox;