Build the playable map from the atlas

Stage 1 of the world-building plan, done in #100: the atlas is baked into static files (map.bin, statics.bin, world.json), the server plays them (--world) with demand-driven regions, the client loads the same bake. Ground, rivers, lakes, roads (fords and causeways for now), town terraces with plaza, lanes and plots, nature by biome, bandit camps. Towns, bridges, forts and dungeon entrances are the next tasks.