Remove null-based accumulators in Household.step

[speedcom]

Summary

• Replace 8 nullable Array[Double] accumulators with a single Option[Tuple8] (perBankArrays)
• Replace br != null / perBankInc != null checks with bankRates match and perBankArrays.foreach
• No behavioral change — same logic, null-free

Test plan

• sbt compile — zero warnings
• sbt test — all 1251 tests pass