feat: 新增 LibreOffice soffice DOCX 解析器

- 新增 scripts/readers/docx/libreoffice.py
- 在 MarkItDown 之后、python-docx 之前插入解析器
- 新增 tests/test_readers/test_docx/test_libreoffice.py
- 更新 openspec/specs/docx-reader/spec.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/readers/docx/__init__.py

@@ -10,6 +10,7 @@ from . import docling
 from . import unstructured
 from . import markitdown
 from . import pypandoc
+from . import libreoffice
 from . import python_docx
 from . import native_xml
 
@@ -19,6 +20,7 @@ PARSERS = [
     ("unstructured", unstructured.parse),
     ("pypandoc-binary", pypandoc.parse),
     ("MarkItDown", markitdown.parse),
+    ("LibreOffice", libreoffice.parse),
     ("python-docx", python_docx.parse),
     ("XML 原生解析", native_xml.parse),
 ]