Mutation testing on the money code only

A full mutation run takes nine hours; a run over one directory takes eleven minutes and answers the question that matters.

$ vendor/bin/infection --only-covered 
    --filter=src/Domain/Pricing --min-msi=85

  Mutation Score Indicator (MSI):  71%
  Mutations:  412
  Killed:     293
  Escaped:     94
  Errors:      25

# line coverage on the same directory: 94%

The gap between 94% coverage and 71% mutation score is the number worth internalising — nearly a quarter of the mutations survived code that was fully executed by tests. Four of the ninety-four were genuine gaps and the rest were equivalent mutants or assertions about things that do not matter, which is the ratio that makes this a periodic exercise rather than a gate.