Implement Calculator Pattern Task

Problem

A Ruby team needs help with a task in this area:

Use when building variant-based calculators with SERVICE_MAP routing via Factory.for(entity) (ONLY permitted entry point — direct instantiation FORBIDDEN), BaseService defines calculate→compute_result if should_calculate?, NullService has should_calculate?=false+compute_result=nil, Concrete services override both methods and MUST call super in should_calculate? — and each component is tested in order (write spec → run → verify Red → implement → run → verify Green before next component, without collapsing NullService and concrete into one step).

The team has asked for a concise implementation artifact that a reviewer can inspect without needing to observe the agent's process.

Output

Create answer.md with:

• a short plan for the work
• the concrete Ruby-oriented artifact or recommendation
• the verification steps or quality gates that should be run
• any assumptions that affect the result