EqualitySaturationSafeRuleCount

Test-only visibility into how many rules EqualitySaturation(AngouriMath.Core.Budgets.WorkBudget,AngouriMath.Core.CostModel) currently
draws from -- the field itself is private on a
private nested class, which a test in another assembly cannot reach
any other way. Exists so a collapse back toward the old
all-Unknown state — the rule population silently
shrinking to nothing, the way it once did before this was measured — fails a test
instead of going unmeasured again.