implement-calculator-pattern examples

1. Factory usage example (Ruby)

calculator = StrategyFactory.for(account: account)
result = calculator.calculate(amount: 1000)

2. Null calculator behavior

NullCalculator returns zeroed results and never raises, safe to use as fallback:

{ value: 0, breakdown: {}, warnings: [] }

3. Lookup order

• The factory tries strategies in lookup_order and falls back to null_calculator if none match.

1. Required verification shape

• Factory, BaseService, NullService, and concrete services each get their own RED command and GREEN checkpoint.
• Concrete services mirror contexts for named variant, inactive plan, nil plan, and unknown variant when applicable.
• BaseService raises NotImplementedError, "#{self.class}#compute_result must be implemented" until a concrete service implements the calculation.