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.

tests/test_build_library.py

@@ -0,0 +1,61 @@
+import json
+import tempfile
+import unittest
+from pathlib import Path
+
+from textbook.build_library import build
+
+
+def write_jsonl(path, records):
+    path.write_text('\n'.join(json.dumps(r) for r in records) + '\n', encoding='utf-8')
+
+
+class BuildLibraryTests(unittest.TestCase):
+    def test_build_includes_only_solved_theorems_and_hoists_shared_imports(self):
+        with tempfile.TemporaryDirectory() as directory:
+            directory = Path(directory)
+            dataset = directory / 'dataset.jsonl'
+            results = directory / 'results.jsonl'
+            write_jsonl(dataset, [
+                {"id": "solved-one", "statement": "True", "prelude": "import Mathlib",
+                 "source_label": "Theorem 1", "natural_language": "trivial"},
+                {"id": "unsolved-one", "statement": "False", "prelude": "import Mathlib",
+                 "source_label": "Theorem 2", "natural_language": "not actually true"},
+            ])
+            write_jsonl(results, [
+                {"id": "solved-one", "success": True,
+                 "attempts": [{"proof": "trivial", "success": True, "diagnostics": ""}]},
+                {"id": "unsolved-one", "success": False, "attempts": []},
+            ])
+
+            text, solved, skipped = build(dataset, results)
+
+            self.assertEqual(solved, ['solved-one'])
+            self.assertEqual(skipped, ['unsolved-one'])
+            self.assertIn('import Mathlib', text)
+            self.assertEqual(text.count('import Mathlib'), 1)  # hoisted once, not per-theorem
+            self.assertIn('theorem solved_one : True := by', text)
+            self.assertNotIn('unsolved_one', text)
+
+    def test_build_keeps_non_import_prelude_attached_per_theorem(self):
+        with tempfile.TemporaryDirectory() as directory:
+            directory = Path(directory)
+            dataset = directory / 'dataset.jsonl'
+            results = directory / 'results.jsonl'
+            write_jsonl(dataset, [
+                {"id": "custom-def", "statement": "myPred 1", "prelude": "def myPred (n : Nat) := True",
+                 "source_label": "Theorem 3", "natural_language": ""},
+            ])
+            write_jsonl(results, [
+                {"id": "custom-def", "success": True,
+                 "attempts": [{"proof": "trivial", "success": True, "diagnostics": ""}]},
+            ])
+
+            text, solved, skipped = build(dataset, results)
+
+            self.assertIn('def myPred (n : Nat) := True', text)
+            self.assertEqual(solved, ['custom-def'])
+
+
+if __name__ == '__main__':
+    unittest.main()