[S3-036] Implement exploration and player-specific visibility

Accidental duplicate of https://github.com/stacksjs/the-settlers-iii-remake/issues/38, 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 S3-036 in https://github.com/stacksjs/the-settlers-iii-remake/issues/38.