savingsPercent
fun savingsPercent(baseMicros: Long, basePeriod: String?, comparedMicros: Long, comparedPeriod: String?): Int
Percentage saved by paying comparedMicros per comparedPeriod instead of baseMicros per basePeriod, normalized to a common cadence.
Typical use is "yearly vs monthly": pass the monthly plan as the base. Returns 0 when either side is unusable (missing price, unparseable period, zero-length period) or when the compared plan is not cheaper.
Return
whole-percent saving in 0..100.