[ty] Preserve declaration order when synthesizing class fields

[charliermarsh]

Summary

Prior to this change, field synthesis could accidentally follow symbol-table order instead of class-body declaration order in pathological cases like:

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 astral-sh/ty#3493.