De-duplicate Spring profile configs, and prove the strip changed nothing

Strip an `application-<profile>.yml` down to the values that actually differ from `application.yml`, and prove the change altered nothing. Use when: (1) a Spring Boot or Grails repo has profile configs that are near-copies of the base config and you cannot tell which lines are real overrides, (2) you are about to change a value in the base config and need to know which profiles silently inherit it, (3) a staging/production config drifted because someone edited one copy of a duplicated key, (4) you want to delete config duplication but cannot risk changing runtime behaviour, (5) you need to decide whether a duplicated value should be deleted or deliberately kept. Covers th…