feat(docreader): add structured DOCX table extraction

[langcaiye]

Summary

• Add structured DOCX table extraction from original Word table XML
• Preserve gridSpan/vMerge merge semantics and render row-level key/value text
• Append structured table text to DOCX MarkItDown output with a safe fallback
• Cover continuation tables, context rows, notes, header detection, and merge recovery in tests

Tests

• PYTHONPATH=. docreader/.venv/bin/python -m unittest docreader.tests.test_docx_table_parser docreader.tests.test_config
• PYTHONPATH=. docreader/.venv/bin/python docreader/scripts/parse_local.py /Users/langcaiye/Downloads/言语处理器常见故障处理方法.docx --out /tmp/speech_processor_structured.md
• git diff --check