This fixes https://github.com/astral-sh/ty/issues/1736 where recursive
generic protocols with growing specializations caused a stack overflow.
The issue occurred with protocols like:
```python
class C[T](Protocol):
a: 'C[set[T]]'
```
When checking `C[set[int]]` against `C[Unknown]`, member `a` requires
checking `C[set[set[int]]]`, which requires `C[set[set[set[int]]]]`,
etc. Each level has different type specializations, so the existing
cycle detection (using full types as cache keys) didn't catch the
infinite recursion.
The fix introduces `TypeRelationKey`, an enum that can be either a full
`Type` or a `ClassLiteral` (protocol class without specialization). For
protocol-to-protocol comparisons, we use `ClassLiteral` keys, which
detects when we're comparing the same protocol class regardless of
specialization. When a cycle is detected, we return the fallback value
(assume compatible) to safely terminate the recursion.
c88e1e40ab