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## Summary
Track the reachability of nested scopes within their parent scopes. We
use this as an additional requirement for emitting
`unresolved-reference` diagnostics (and in the future,
`unresolved-attribute` and `unresolved-import`). This means that we only
emit `unresolved-reference` for a given use of a symbol if the use
itself is reachable (within its own scope), *and if the scope itself is
reachable*. For example, no diagnostic should be emitted for the use of
`x` here:
```py
if False:
x = 1
def f():
print(x) # this use of `x` is reachable inside the `f` scope,
# but the whole `f` scope is not reachable.
```
There are probably more fine-grained ways of solving this problem, but
they require a more sophisticated understanding of nested scopes (see
#15777, in particular
https://github.com/astral-sh/ruff/issues/15777#issuecomment-2788950267).
But it doesn't seem completely unreasonable to silence *this specific
kind of error* in unreachable scopes.
## Test Plan
Observed changes in reachability tests and ecosystem.