[ty] Preserve constraints from bound receivers (#26776)
## Summary
Binding a method currently removes its receiver parameter and loses any
relationship between the concrete receiver and an explicit receiver
annotation. For a method like `[T](self: T, value: T) -> T`, that lets
later callable comparisons choose `T = int` even when the method is
bound to an unrelated class.
This PR retains that receiver relation on the bound signature while the
signature is freshened, specialized, and compared. The relation
participates in existential callable-local inference, and the runtime
receiver remains distinct from the type used to replace `typing.Self`,
which is necessary for class methods and constructors.
For example, we previously accepted this assignment:
```python
from typing import Callable
class C:
def method[T](self: T, value: T) -> T:
return self
method: Callable[[int], int] = C().method
```
Binding `C().method` requires `C` to be assignable to `T`. We can
therefore no longer choose `T = int` merely to satisfy the target
callable, and now report this as an invalid assignment
(This pass supports unbounded receiver TypeVars, but declared bounds and
constraints are deferred for now... Tacking those on led to ecosystem
failures around recursive protocols.)
