refactor: 重构 Reader 内部工具函数到独立模块

- 新增 scripts/readers/_utils.py 作为 Reader 内部共享工具模块
- 将 parse_with_markitdown 等函数从 core/markdown.py 迁移到 _utils.py
- 函数重命名：parse_with_xxx → parse_via_xxx，_unstructured_elements_to_markdown → convert_unstructured_to_markdown
- 更新 17 个 Reader 实现文件的 import 路径
- 从 core/__init__.py 移除已迁移函数的导出
- 新增测试文件 tests/test_readers/test_utils.py
- 新增 spec 文档 openspec/specs/reader-internal-utils/spec.md

这次重构明确了模块边界：core/ 提供公共 API，readers/_utils.py 提供 Reader 内部工具

scripts/core/__init__.py

@@ -8,18 +8,12 @@ from .exceptions import (
     DownloadError,
 )
 from .markdown import (
-    parse_with_markitdown,
-    parse_with_docling,
-    build_markdown_table,
-    flush_list_stack,
-    safe_open_zip,
     normalize_markdown_whitespace,
     remove_markdown_images,
     get_heading_level,
     extract_titles,
     extract_title_content,
     search_markdown,
-    _unstructured_elements_to_markdown,
 )
 from .parser import parse_input, process_content, output_result
 
@@ -29,18 +23,12 @@ __all__ = [
     "ReaderNotFoundError",
     "ParseError",
     "DownloadError",
-    "parse_with_markitdown",
-    "parse_with_docling",
-    "build_markdown_table",
-    "flush_list_stack",
-    "safe_open_zip",
     "normalize_markdown_whitespace",
     "remove_markdown_images",
     "get_heading_level",
     "extract_titles",
     "extract_title_content",
     "search_markdown",
-    "_unstructured_elements_to_markdown",
     "parse_input",
     "process_content",
     "output_result",