Support lower generic bounds

```py
class A[LowerBound[T, int]]: ...

A[object] # ok
A[int] # ok
A[bool] # nah
```

# mypy in real life

```py
class list[T]:
def copy[R: lower=T](self) -> list[R]: ...
a: list[int]
b: list[object] = a.copy() # okay, R is inferred as object
```

No file or test path is named. Start by locating the implementation and tests for generic type parameters and constraints, then compare existing bound handling with the LowerBound[T, int] and lower=T examples. Done means the examples produce the stated accepted and rejected results, with regression coverage.

Written by the indexing model from the issue text.