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/mir/builder.rs

@@ -434,6 +434,14 @@ impl MirBuilder {
         }
 
         if let Some(ref mut function) = self.current_function {
+            // Pre-capture branch/jump targets for predecessor update after we finish
+            // mutably borrowing the current block.
+            let (then_t, else_t, jump_t) = match &instruction {
+                MirInstruction::Branch { then_bb, else_bb, .. } => (Some(*then_bb), Some(*else_bb), None),
+                MirInstruction::Jump { target } => (None, None, Some(*target)),
+                _ => (None, None, None),
+            };
+
             if let Some(block) = function.get_block_mut(block_id) {
                 if utils::builder_debug_enabled() {
                     eprintln!(
@@ -484,12 +492,23 @@ impl MirBuilder {
                     );
                 }
                 block.add_instruction(instruction);
-                Ok(())
-            } else {
-                Err(format!("Basic block {} does not exist", block_id))
+                // Drop the mutable borrow of `block` before updating other blocks
+                }
+            // Update predecessor sets for branch/jump immediately so that
+            // debug_verify_phi_inputs can observe a consistent CFG without
+            // requiring a full function.update_cfg() pass.
+            if let Some(t) = then_t {
+                if let Some(succ) = function.get_block_mut(t) { succ.add_predecessor(block_id); }
             }
+            if let Some(t) = else_t {
+                if let Some(succ) = function.get_block_mut(t) { succ.add_predecessor(block_id); }
+            }
+            if let Some(t) = jump_t {
+                if let Some(succ) = function.get_block_mut(t) { succ.add_predecessor(block_id); }
+            }
+            Ok(())
         } else {
-            Err("No current function".to_string())
+            Err(format!("Basic block {} does not exist", block_id))
         }
     }