[ty] Infer generic class pattern capture types

[charliermarsh]

Summary

When a class pattern matches a generic subclass through a specialized base-class subject, captures declared on the subclass previously fell back to Unknown or object. Narrowing retained the base specialization, but member lookup still used an unspecialized pattern class.

This infers the pattern subclass's type arguments by solving the mapping from its generic base to the subject class. We use the result only when every pattern-class type variable has one exact invariant solution; covariant bases and unconstrained parameters keep the existing conservative fallback:

class Base[T]: ...

class Child[T](Base[T]):
    item: T

def f(value: Base[int]) -> None:
    match value:
        case Child(item=item):
            reveal_type(item)  # int

Closes astral-sh/ty#3879.