Strategy + Factory + Null Object Calculator Pattern

Implements a variant-based calculator system with a single entry point, concrete strategies, and a no-op fallback (Null Object).

Core principle: One API for the client: Calculator::Factory.for(entity).calculate. The factory picks the strategy; NullService handles unknown variants safely.

HARD-GATE: Tests Gate Implementation

EVERY component (Factory, BaseService, NullService, concrete services) MUST have
its test written and validated BEFORE implementation.
  1. Write the spec for the component (contexts per variant)
  2. Run the spec — verify it fails because the component does not exist yet
  3. ONLY THEN write the component implementation
  4. Repeat for each component: Factory → BaseService → NullService → Concrete
See rspec-best-practices for the full gate cycle.

Quick Reference