Implement Unpack of TypeVars for **kwargs inference
Allow `**kwargs: Unpack[K]` where K is a TypeVar bound to TypedDict.
This enables inferring TypedDict types from keyword arguments at call sites.
Example:
def f[K: BaseTypedDict](**kwargs: Unpack[K]) -> K: ...
result = f(x=1, y="hello") # K inferred as TypedDict({'x': int, 'y': str})
Changes:
- semanal.py: Accept TypeVar with TypedDict bound in remove_unpack_kwargs(),
keep UnpackType for constraint inference
- semanal_typeargs.py: Allow TypeVar with TypedDict bound in visit_unpack_type()
- constraints.py: Generate TypedDict constraints from actual kwargs
- types.py: Handle TypeVar in with_unpacked_kwargs()
- checkexpr.py: Re-expand kwargs after TypeVar inference
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
