make `allow-redefinition` better / require annotation

```py
a: int | str = 1
reveal_type(a) # int | str
a = ""
reveal_type(a) # str
```
This is actually redefining a as `str`, not reassigning as part of the union.

What if it always needed a type annotation to redefine:
```py
a: int | str = 1
a = str(a) # no redefine
a = 1 # no error, and no redefine 👍
```
should need:
```py
a: int | str = 1
a: str = str(a)
a = 1 # error 👍
```

Using `Infer` would also be usable here no announce a redefinition
```py
a = 1
a: Infer = str(a)
```
Perhaps even a separate annotation `Redefine`

Start by tracing the existing `allow-redefinition` handling in the type checker. Compare the issue's annotation proposals and examples, then clarify which syntax and semantics are intended; done means the chosen rule distinguishes ordinary reassignment from explicit redefinition and has agreed behavior for the shown cases.

Written by the indexing model from the issue text.