Interface fields declare a narrower post_process return type than ip_interface() gives, as two standing mypy [return-value] errors

[JarryShaw]

Summary

Both interface fields' post_process return the result of
ipaddress.ip_interface(...), which is typed IPv4Interface | IPv6Interface,
against declared return types that admit only one of the two. mypy reports it as
two [return-value] errors, and the declared annotation is a promise the code
does not keep.

Measured

On fa128959e, with the Makefile's own invocation:

pcapkit/corekit/fields/ipaddress.py:216:16: error: Incompatible return value type
    (got "IPv4Interface | IPv6Interface", expected "IPv4Interface")  [return-value]
pcapkit/corekit/fields/ipaddress.py:291:16: error: Incompatible return value type
    (got "IPv4Interface | IPv6Interface", expected "IPv6Interface")  [return-value]

The declarations are at pcapkit/corekit/fields/ipaddress.py:194
(IPv4InterfaceField.post_process -> 'IPv4Interface') and :262
(IPv6InterfaceField.post_process -> 'IPv6Interface').

Why it is worth fixing rather than ignoring

At runtime the value is always the declared type, because the octet count fixes
the version — so this is not a live bug. But it is two of the repository's
standing mypy errors, and it is the kind that hides a real one: a caller reading
the annotation is entitled to assume the narrower type, and the checker cannot
tell the difference between this and a genuine mismatch.

Both sites already validate the version immediately afterwards
(val.version != self.version raising FieldValueError), so the narrowing is
available — it just is not expressed in a way mypy can follow.

Suggested direction

Either construct the concrete class directly rather than going through the
version-dispatching ip_interface() factory, or narrow after the existing
version check in a form mypy accepts. Not a # type: ignore: the annotation
is currently the inaccurate part, and silencing the checker would leave it that
way.

Provenance

Flagged by the agent that produced #470 and verified independently on main
before filing. Pre-existing and unrelated to that PR, which left both errors in
place with only their line numbers shifting.