Show cleartext representation of wallet policy, when possible

Wires the cleartext engine into the register-wallet flow: the handler
computes the confusion score and, when below the threshold, renders the
cleartext spending-path lines shown before the raw descriptor template.

For multisig wallet policies specifically, this also simplifies the UX by
omitting the raw descriptor template altogether. This is safe for such
simple policies, with very little ambiguity. In the long term, we might be
able to extend this to more complex wallet policies, but that needs to be
done intentionally, and possibly be an opt-in feature that is explicitly
requested by software wallets that adapt their own UX accordingly.

Unit tests of register_wallet had to be adapted slightly to take into
account that the descriptor is not always shown.