RuleBasedFactorizationAtLevel​(System.​Int32)

Separate because Simplify offers a factorisation as a candidate and its
cost model decides. The metric prefers the expanded form — x ^ 6 - 1 rates 12
expanded against 58 factored — so a factored candidate wins only where the two are
closest, and those turn out to be the places a factored answer is least wanted:
x ^ 3 / 3 + x ^ 2 / 2 becomes (3 + 2 * x) * x ^ 2 / 6, an antiderivative
in a form nobody writes. Offering the layer to that search is
#746 tier 2's
pluggable cost model rather than
#1018.