discovery: stop direct use of attribute of partialdiscovery
Instead of accessing `undecided` directly for ui display purposes,
we introduce a `stats()` method that could be extended in the future with
more interesting information.
This is in preparation for a forthcoming Rust version of this object.
Indeed, attributes and furthermore properties are a bit complicated for
classes in native code.
We could go further and rename `undecided` to mark it private, but `_undecided`
is already taken as support for `_undecided` lazyness.
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