Records in depth: validation, defensive copies and withers #93

Goal: go beyond the basic PairRecord test and show how records are used in practice.

Show:

• compact canonical constructor for validation and normalization
• defensive copy of a mutable component (List.copyOf)
• "wither" methods returning a modified copy
• pitfall: array components and generated equals

Where: FirstSteps/src/test/java/pl/mperor/lab/java/idiom