persistent_fractional: strengthen like bi.persistent_sep_dup

In 13c5c1ad I strengthened bi.persistent_sep_dup
to support propositions that are persistent and either affine or absorbing; do
the same for `persistent_fractional`.
2 jobs for !717 with persistent-fractional in 11 minutes and 37 seconds (queued for 10 seconds)
detached
Status Job ID Name Coverage
  Build
passed build-coq.8.12.2 #135345
fp

00:10:16

passed build-coq.dev #135344
fp

00:11:25