Support usage of slack variables (lower bound emulation)

Nobody has claimed this yet.

Context: https://github.com/python/typing/issues/674

[Slack variables](https://en.wikipedia.org/wiki/Slack_variable) would allow us to emulate lower bounds on type variables, which is relevant for methods like `dict.__or__` or sequence concatenation, when an outer context is present.

Code sample in [pyright playground](https://pyright-play.net/?code=GYJw9gtgBALgngBwJYDsDmUkQWEMoByApgG5EgA0UAhgM63kwD68CRVIpR1ANi4kQBQggMY86tKADUiIgNoAVALoAuKFADEmFCWogk1FPlRQFg9eoAmRYFCZIYRCLQAUDHsACUUALQA%2BKB4kWhhFVSgAOijzC2tbJmoENhRLNyIPKgcnNQVvf0IwFCI1KIjhOKgRQpFqZjIRWkUARioFACYqJgUmWnERAGsAXmIyECUXGPUeGxg1GXkFJqUKSah9NAALWelZRTblwTyA%2BeaoAB9TNvO7bt7qAfDNKAAeHx8oAGU%2B-qhdfWoAEbTKAQaj9IiSTgwACuIBQNCgtGhbDwAigYFsi2u7VWUNh8PmLk8wjEEigAEEStFSfQoAAhKllGmSADCjPKNlgELqsiYVRQNRgLmmwG2J3JyzWSE2Yt2dKURwKRRUqy0AHcHBswND8NrHCBKoVHAAPGBUGAbIiwNHBKAITgiYJCCw0eiMfhsFz8wVMequEVmqUyzxUcXXeXeJ6AUHJAPB-qvROvIhqMRFNNB4PDAaskGusKFQGEs0KtMDAiO%2Bvz0BiBENWTAADHNduTw0oXVBBsmfX7hTMONKtt4tLG600m-IWxc6dcWW3O97ar7ZP6%2B0HB9G4y6mG1x3IwACAFayGBtizz6qLnsB-vBp6AGXI40A)

```python
from typing import Never, assert_type, reveal_type

class Vec[T]: # invariant in T
def _items(self) -> list[T]: ...
def _append(self, item: T) -> None: ...

def concat_vecs[T1, T2, _T_slack=Never](
left: Vec[T1],
right: Vec[T2],
) -> Vec[T1 | T2 | _T_slack]: # <-- Slack variable makes return a supertype of T1 | T2
return Vec()

class A: ...
class B: ...
class C: ...

def test_vec_concat(left: Vec[A], right: Vec[B]) -> None:
# without outer context, the type is precise
assert_type(concat_vecs(left, right), Vec[A | B]) # ✅️
# outer context allows widening due to slack variables
_0: Vec[A | B] = concat_vecs(left, right) # ✅️
_1: Vec[A | B | C] = concat_vecs(left, right) # ✅️
_2: Vec[object] = concat_vecs(left, right) # ❌️

```

In the example above, `pyright` fails to find the solution (`T1=A`, `T2=B`, `_T_slack=object`) in the last test case.

Reproduce the concat_vecs example in the linked pyright playground and read the referenced typing issue for the proposed slack-variable behavior. Done means the final Vec[object] assignment succeeds while the no-context assertion remains precise, with the relevant behavior covered by tests.

Written by the indexing model from the issue text.