Copyright year calculation with merge commits

Copyright year calculation currently takes the date of the merge commit, even if that particular file merged cleanly.

It causes a specific failure in spotlessCheck (I used Gradle plugin, but the issue is likely not related to Gradle): when branches being merged were changed in the previous year (e.g. 2025) and both had correct copyright end year, checked by Spotless at the time of their respective commits, and when the merge itself happens in the new year (e.g. 2026), Spotless insists that the 2025 copyright year from the clean merge is now incorrect and requires a manual change to 2026.

The expected outcome is that the clean, non-conflicting merge commit does not advance the copyright end year.
Merge commit that actively changes a file (to resolve conflicts or otherwise) should still advance the end year.

Comienza localizando el cálculo del año de copyright y las pruebas o fixtures que cubren los merge commits; el issue no menciona archivos ni puntos de entrada. Reproduce la comprobación de Spotless descrita con ramas modificadas en años diferentes y un merge limpio. Se considera terminado cuando un merge limpio no adelanta el año final, mientras que un merge que modifica el archivo sí lo sigue haciendo.

Escrito por el modelo de indexación a partir del texto del issue.