Report a refuted instance without building a vtree over it

[guyvdbroeck]

FrontendSession::build_run short-circuits only when preprocessing leaves no
variables. A refutation is not that case: reduced.cnf is by design an explicit
contradiction over the ORIGINAL variable count, because DIMACS cannot portably
spell the empty clause. So a refuted run went on to select a spec and build a
full vtree over a formula whose answer the record already gives —
PreprocessRecord::unsat says "The count is 0 and no compilation is needed".
Construction gets a share of the wall that is left (by default a third of it, at
least 90 s and never past the run deadline), so the longer the caller's budget,
the more of it a refutation spends building a vtree nothing will compile.

RunVtree gains a third variant, Refuted, and build_run returns it as soon
as the record says the instance was refuted — before selection, before
construction. built() is None for it, so write_to_dir writes the bundle
alone and names no vtree files, and the command-line summary reports the verdict
where the vtree lines would be.

API note: RunVtree is public and this is a new variant, so an exhaustive match
on it needs the new arm.