add a opt-in undriven signals report available at najaeda python level

[xtofalex]

This comes from a discussion in another issue. The idea is to provide a user report or functionality for detecting undriven signals.

On undriven signals: agreed, opt-in. Unconditional warnings would drown here —
this design uses don't-care assignment deliberately. A naive scan also flags
every top-level input, which has no leaf driver by construction.

Most useful to me, in order:

1. A summary count in normal output — nets with loads and no drivers,
   top-level inputs excluded. A number rather than a warning, so the
   false-positive concern doesn't apply. Its value is comparative: across tool
   versions, or between the two sides of an equivalence check. A count moving
   when the design didn't is the signal.
2. The opt-in detailed report, for when that number looks wrong.

Originally posted by @ovebryne in #430