## Summary

Like Mypy and Pyright, we now ignore annotations on non-name (i.e.,
subscript and attribute) targets, and emit a diagnostic for the invalid
annotation -- with the exception of annotated assignments on `self` and
class attributes (i.e., receivers).

Closes astral-sh/ty#509.

)

## Summary

We now emit `invalid-type-form` for an invalid annotation on a non-name
target (#25324)... But it turns out that we still allow that annotation
to participate in downstream operations, since we create an
annotated-assignment definition for it during the indexing pass.

For example:

```python
def accepts_inner(inner: dict[str, int]): ...

def _(x: dict[str, object]):
    x["inner"]: dict[str, float | str] = {"a": 1, "b": "a"}  # error: [invalid-type-form]
    x["inner"]["c"] = 1.0  # error: [invalid-assignment] because we infer from the RHS, not the annotation
    x["inner"] = {"inner": {"a": 1}}
    accepts_inner(**x["inner"])  # ok
```

The rejected annotation should not widen the dictionary assigned to
`x["inner"]`, nor should it constrain the later replacement. (The same
issue applies to rejected attribute annotations, like `obj.inner: T`.)

We now represent each syntactically indexed declaration outcome as
either `Declared(...)` or `Rejected`. Declaration consumers only use
`Declared(...)`.