fix: don't leave the engine in classical mode after a compare query

[nikolauspschuetz]

_plequal (the shared engine behind compare, compare_nouns, compare_verbs, compare_adjs) snapshots classical_dict to restore it, but the three early returns (eq/p:s/s:p) skip the restore, so a comparison leaves the engine in classical mode:

p = inflect.engine()
p.compare("formula", "formulae")   # 's:p'
p.plural("formula")                # 'formulae'  (was 'formulas')

A comparison query shouldn't mutate the engine — classical mode is controlled only by classical(). Wrapping the loop in try/finally restores the snapshot on every path. Added a regression test.