[S3-074] Complete skirmish setup, difficulty, and random maps

Accidental duplicate of https://github.com/stacksjs/the-settlers-iii-remake/issues/76, created on 2026-10-22 when an interrupted roadmap publish (the disk filled during the run) could not see the existing issues. Closed without work; track S3-074 in https://github.com/stacksjs/the-settlers-iii-remake/issues/76.