Add textbook formalization pipeline (extract -> LLM formalize -> elaboration check -> library assembly)

- extract_section.py: slice a book section out of a pdftotext -layout dump and
  list its labeled Theorem/Proposition/Lemma/Corollary/Definition items.
- formalize.py: LLM-formalize a section into Lean 4 candidate statements
  (preferring Mathlib vocabulary) plus a definitions manifest.
- check_statements.py: elaboration-only sanity check against a real
  Mathlib-backed Lean project (must run outside the Cowork sandbox, which has
  no Lean toolchain and can't run macOS binaries anyway).
- build_library.py: assemble solved lean-bench results into a reusable .lean
  file, reusing each winning proof verbatim.
- llm.py: standalone OpenAI-compatible chat client + .env loader, kept
  independent of lean_agent so this pipeline works without it installed.
- Source PDFs and raw/layout text dumps are gitignored (copyrighted material);
  only code and the final short Lean statement datasets are tracked.

README.md

@@ -26,3 +26,9 @@ python3 -m unittest discover -s tests -v
 ```
 
 mock 返回固定证明，仍由真实 Lean 检查。真实 LLM 需配置 endpoint/model 或适配器。
+
+## 教科书形式化流水线
+
+`textbook/` 下有一套从教材 PDF 生成 lean-bench 数据集的工具（LLM 辅助抽取
+Theorem/Proposition/Lemma/Corollary，配 Definition 清单，事后把证出来的结果
+组装成可复用的 Lean 库文件），用法见 [textbook/README.md](textbook/README.md)。