Unable to add single address without having the xpub

I have an old address which is simply just a single public key and its corresponding private key. I do not have the xpub used to generate it.

I want to add a watch-only wallet in Specter where I just give it the public key for one address and can then see the transactions for that address.

This doesn't seem to be possible as Specter requires you to enter an xpub.

Can the ability to add individual public/private keys be added?

No files, tests, or entry points are named. Start by tracing how Specter currently creates watch-only wallets and validates xpub input, then identify where a single address and its keys would fit. Done means a user can add one address without an xpub and view that address's transactions, with tests covering the new wallet path.

Written by the indexing model from the issue text.