test: green up after Phase 2 merge for #26\n\n- Fix AST tests (is_override)\n- Align MIR builder test with current If AST\n- Adjust ResultBox API usage in tests\n- Add len() helpers to ArrayBox/BufferBox for tests

tests/development/test_implicit_override_error.nyash

@@ -0,0 +1,19 @@
+
+// 暗黙オーバーライド禁止のテスト
+box Parent {
+    init { }
+    
+    method() {
+        return "Parent method"
+    }
+}
+
+box Child from Parent {
+    init { }
+    
+    // overrideなしで同名メソッド定義（エラーになるはず）
+    method() {
+        return "Child method"
+    }
+}
+