F841 does not have branch analysis

### Summary

F841 does not report variables that are unused in certain branches. Below is a short example ([playground](https://play.ruff.rs/0e1d38e6-17b0-4355-a37a-089d454de311)) showing the issue

```python
def func(x):
if x:
io = ""
else:
io = "foo"
print(io)
```

This issue was [originally reported](https://github.com/PyCQA/pyflakes/issues/758) to pyflakes and closed as duplicate of https://github.com/PyCQA/pyflakes/issues/715.

They mention is unlikely they will implement branch analysis, but ruff already has (I guess) the machinery needed to support this use case.

### Version

v0.11.0 (playground)

Start with the linked playground example and inspect Ruff's F841 rule and the existing branch-analysis machinery. Reproduce the example, then determine the expected diagnostic for the branch-local unused variable; done means F841 reports the appropriate unused variable without regressing existing behavior.

Written by the indexing model from the issue text.