infer type-guards

```py
def guard(x: object) -> bool: # inferred as "x is int"
return isinstance(x, int)

def is_even(x: object) -> bool: # inferred as "x is int if True else False"
return isinstance(x, int) and x > 0

Start with the two Python examples in the issue and trace the type-inference paths for functions returning isinstance checks and compound conditions. Done means the checker infers the illustrated type-guard behavior for both guard and is_even, with tests covering those results.

Written by the indexing model from the issue text.