support `Truthy`/`Falsy` types

### Describe the Bug

```py
class A: ...

def f(a: A):
assert a
reveal_type(a) # A, expected A & Not[Falsey]
```

### Sandbox Link

_No response_

### (Only applicable for extension issues) IDE Information

_No response_

Start by locating Pyrefly's handling of assert-based type narrowing and the tests covering truthiness or falsiness. Trace how the asserted value is represented, then add coverage for the example where `reveal_type(a)` should become `A & Not[Falsey]`; done means the narrowing is reported as expected.

Written by the indexing model from the issue text.