Install guide: verify step calls `nu.__version__`, which does not exist

[tamohannes]

docs/how-to/install.md gives this as the verify step:

uv run python -c "import nu; print(nu.__version__)"

It fails on a correctly installed tree:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
AttributeError: module 'nu' has no attribute '__version__'

src/nu/__init__.py does not define __version__; the version lives only in pyproject.toml (version = "0.1.0").

This is the step a user runs to confirm the install worked, so it reads as a broken install rather than a broken doc.

Two ways to fix

Add the attribute (keeps the doc as written, and nu.__version__ is what people will reach for anyway):

# src/nu/__init__.py
from importlib.metadata import version as _version

__version__ = _version("nu")

Or change the doc to something that already works:

uv run python examples/basic.py   # prints 42

The first seems better since the docs already promise the attribute, but the second is zero risk.

[tamohannes]

Fix for this is in the same local branch as #2: nu.__version__ defined via importlib.metadata, with a PackageNotFoundError fallback for source trees. See #2 for why it is not a PR yet.