Reintroduce multiple maps: per-map chunk directories + map_t name/mapSetMap

Move chunk assets from a single flat assetsraw/chunks (and assets/chunks)
into per-map assetsraw/maps/<map>/chunks (assets/maps/<map>/chunks), so
terrain mesh/model output no longer collides across maps sharing chunk
coordinates. map_t gains a name[MAP_NAME_MAX] field plus mapSetMap()/
mapIsLoaded() (checks name[0] == '\0') in place of the old bool loaded
flag; mapInit() now just resets state and mapSetMap() does the chunk
grid load. Updated the chunk asset tool and the map editor's dev
server/client to match.

assetsraw/maps/overworld/map.json

@@ -0,0 +1 @@
+{}