[ty] Preserve declaration order when synthesizing class fields (#25249)
## Summary
Prior to this change, field synthesis could accidentally follow
symbol-table order instead of class-body declaration order in
pathological cases like:
```python
from dataclasses import InitVar, dataclass
from ty_extensions import Top
@dataclass
class C:
a: Top[int]
int: InitVar[int] = 0
```
Here, `int` can be interned before its own declaration because it
already appears in `Top[int]`.
This PR carries a stable declaration ordinal through declaration lookup
and uses it when building `own_fields()`.
Closes https://github.com/astral-sh/ty/issues/3493.
ruff
f242f371 - [ty] Preserve declaration order when synthesizing class fields (#25249)
