F3 toggles a debug overlay with:
- FPS
- Mouse world/tile coordinates
- Tile type under cursor
- Resources, buildings, crops on hovered tile
- Nisse count broken down by AI state (idle/walking/working/sleeping)
- Active jobs by type (chop/mine/farm)
- Pathfinding visualization: cyan lines + destination highlight
drawn in world space via DebugSystem
Added DebugSystem to GameScene. VillagerSystem exposes
getActivePaths() for the path visualization. JSDoc added to all
previously undocumented methods in VillagerSystem, WorldSystem,
GameScene, and UIScene.
