refactor(vm): Phase 4 - Void Guard統一化（重複削減）

boxes_void_guards.rs新規作成で28行の重複を排除

実装内容:
- handle_void_method()ヘルパー関数作成
- 7種類のメソッド(is_eof/length/substring/push/get_position/get_line/get_column)統一
- boxes.rs: 30行→18行（12行削減）
- 重複ブロック2箇所→シングルソース化

効果:
- 保守性向上: 単一の真実の源（Single Source of Truth）
- 可読性向上: 大きなmatchブロック→簡潔なヘルパー呼び出し
- バグ修正容易化: 1箇所修正で全体に反映

テスト: Void.is_eof(), Void.length()正常動作確認

src/backend/mir_interpreter/handlers/boxes.rs

@@ -152,28 +152,16 @@ impl MirInterpreter {
         // e.g., `A or not last.is_eof()` should not crash when last is absent.
         match self.reg_load(box_val)? {
             VMValue::Void => {
-                match method {
-                    "is_eof" => { if let Some(d) = dst { self.regs.insert(d, VMValue::Bool(false)); } return Ok(()); }
-                    "length" => { if let Some(d) = dst { self.regs.insert(d, VMValue::Integer(0)); } return Ok(()); }
-                    "substring" => { if let Some(d) = dst { self.regs.insert(d, VMValue::String(String::new())); } return Ok(()); }
-                    "push" => { if let Some(d) = dst { self.regs.insert(d, VMValue::Void); } return Ok(()); }
-                    "get_position" => { if let Some(d) = dst { self.regs.insert(d, VMValue::Integer(0)); } return Ok(()); }
-                    "get_line" => { if let Some(d) = dst { self.regs.insert(d, VMValue::Integer(1)); } return Ok(()); }
-                    "get_column" => { if let Some(d) = dst { self.regs.insert(d, VMValue::Integer(1)); } return Ok(()); }
-                    _ => {}
+                if let Some(val) = super::boxes_void_guards::handle_void_method(method) {
+                    if let Some(d) = dst { self.regs.insert(d, val); }
+                    return Ok(());
                 }
             }
             VMValue::BoxRef(ref b) => {
                 if b.as_any().downcast_ref::<crate::box_trait::VoidBox>().is_some() {
-                    match method {
-                        "is_eof" => { if let Some(d) = dst { self.regs.insert(d, VMValue::Bool(false)); } return Ok(()); }
-                        "length" => { if let Some(d) = dst { self.regs.insert(d, VMValue::Integer(0)); } return Ok(()); }
-                        "substring" => { if let Some(d) = dst { self.regs.insert(d, VMValue::String(String::new())); } return Ok(()); }
-                        "push" => { if let Some(d) = dst { self.regs.insert(d, VMValue::Void); } return Ok(()); }
-                        "get_position" => { if let Some(d) = dst { self.regs.insert(d, VMValue::Integer(0)); } return Ok(()); }
-                        "get_line" => { if let Some(d) = dst { self.regs.insert(d, VMValue::Integer(1)); } return Ok(()); }
-                        "get_column" => { if let Some(d) = dst { self.regs.insert(d, VMValue::Integer(1)); } return Ok(()); }
-                        _ => {}
+                    if let Some(val) = super::boxes_void_guards::handle_void_method(method) {
+                        if let Some(d) = dst { self.regs.insert(d, val); }
+                        return Ok(());
                     }
                 }
             }

src/backend/mir_interpreter/handlers/boxes_void_guards.rs

@@ -0,0 +1,21 @@
+// Void/VoidBox graceful method guards
+// Extracted from boxes.rs to eliminate duplication (28 lines → 6 lines)
+
+use super::super::VMValue;
+
+/// Handle common methods on Void/VoidBox with graceful fallback values.
+/// Used for short-circuit patterns like `A or not last.is_eof()` where `last` may be absent.
+///
+/// Returns Some(VMValue) if the method is a known void-safe method, None otherwise.
+pub(super) fn handle_void_method(method: &str) -> Option<VMValue> {
+    match method {
+        "is_eof" => Some(VMValue::Bool(false)),
+        "length" => Some(VMValue::Integer(0)),
+        "substring" => Some(VMValue::String(String::new())),
+        "push" => Some(VMValue::Void),
+        "get_position" => Some(VMValue::Integer(0)),
+        "get_line" => Some(VMValue::Integer(1)),
+        "get_column" => Some(VMValue::Integer(1)),
+        _ => None,
+    }
+}

src/backend/mir_interpreter/handlers/mod.rs

@@ -8,6 +8,7 @@ mod boxes_map;
 mod boxes_object_fields;
 mod boxes_instance;
 mod boxes_plugin;
+mod boxes_void_guards;
 mod calls;
 mod externals;
 mod memory;