Lab walkthrough: Library Management Package

Try the spec in the README first. If you're stuck, build it in this order, then compare with lab/.

1. models.py: @dataclass Book (isbn, title, author, issued_to: str | None = None, an available property) and @dataclass Member (member_id, name, borrowed: list[str] = field(default_factory=list)).
2. library.py: class Library holds two dicts, books keyed by ISBN and members keyed by ID. Write add_book (reject duplicates), add_member, issue_book (check availability and a MAX_BOOKS_PER_MEMBER limit), return_book, and search (case-insensitive over title and author). Define BookNotAvailableError here.
3. __init__.py: re-export the public names.
4. Tests first for each rule. After writing each method, add its tests and run uv run pytest -q.
5. storage.py: save(library, path) uses asdict + json.dumps, and load(path) uses Book(**d). Add the round-trip test.
6. ruff check, ruff format, commit, push.

Notice the design choice: Library stores books in a dict keyed by ISBN, so lookups are instant, and it raises errors instead of printing them, so the caller (a CLI today, an API next week) decides how to show them.