fix: reject DXF files missing EOF marker

Add regression coverage for issue #1.

r2c.py

@@ -51,6 +51,8 @@ def read_dxf_bytes(raw):
                 current['section'] = section
     if current:
         records.append(current)
+    if not records or records[-1]['type'] != 'EOF' or records[-1]['tags']:
+        raise ValueError('DXF 종료 표시(EOF)가 없습니다. 파일이 끝까지 저장되었는지 확인해 주세요.')
     entities = [r for r in records if r['section'] == 'ENTITIES']
     blocks, block = {}, None
     for record in records: