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/integer_box.rs

@@ -0,0 +1,82 @@
+//! IntegerBox - Integer values in Nyash
+//!
+//! Implements the core IntegerBox type for 64-bit signed integers.
+
+use crate::box_trait::{NyashBox, BoxCore, BoxBase, StringBox, BoolBox};
+use std::any::Any;
+use std::fmt::{Debug, Display};
+
+/// Integer values in Nyash - 64-bit signed integers
+#[derive(Debug, Clone, PartialEq)]
+pub struct IntegerBox {
+    pub value: i64,
+    base: BoxBase,
+}
+
+impl IntegerBox {
+    pub fn new(value: i64) -> Self {
+        Self {
+            value,
+            base: BoxBase::new(),
+        }
+    }
+
+    pub fn zero() -> Self {
+        Self::new(0)
+    }
+}
+
+impl BoxCore for IntegerBox {
+    fn box_id(&self) -> u64 {
+        self.base.id
+    }
+
+    fn parent_type_id(&self) -> Option<std::any::TypeId> {
+        self.base.parent_type_id
+    }
+
+    fn fmt_box(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(f, "{}", self.value)
+    }
+
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn as_any_mut(&mut self) -> &mut dyn Any {
+        self
+    }
+}
+
+impl NyashBox for IntegerBox {
+    fn to_string_box(&self) -> StringBox {
+        StringBox::new(self.value.to_string())
+    }
+
+    fn equals(&self, other: &dyn NyashBox) -> BoolBox {
+        if let Some(other_int) = other.as_any().downcast_ref::<IntegerBox>() {
+            BoolBox::new(self.value == other_int.value)
+        } else {
+            BoolBox::new(false)
+        }
+    }
+
+    fn type_name(&self) -> &'static str {
+        "IntegerBox"
+    }
+
+    fn clone_box(&self) -> Box<dyn NyashBox> {
+        Box::new(self.clone())
+    }
+
+    /// 仮実装: clone_boxと同じ（後で修正）
+    fn share_box(&self) -> Box<dyn NyashBox> {
+        self.clone_box()
+    }
+}
+
+impl Display for IntegerBox {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.fmt_box(f)
+    }
+}