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.

editor/client/public/map/map.js

@@ -345,12 +345,14 @@
     return promise;
   }
 
+  const MAP_NAME = "overworld";
+
   function chunkPaths(x, y, z) {
     return {
-      raw: `assetsraw/chunks/chunk_${x}_${y}_${z}.json`,
-      dcf: `assets/chunks/${x}_${y}_${z}.dcf`,
-      terrainDmf: `assets/meshes/chunks/chunk_${x}_${y}_${z}_0.dmf`,
-      terrainModel: `assets/models/chunks/chunk_${x}_${y}_${z}_0.json`,
+      raw: `assetsraw/maps/${MAP_NAME}/chunks/chunk_${x}_${y}_${z}.json`,
+      dcf: `assets/maps/${MAP_NAME}/chunks/${x}_${y}_${z}.dcf`,
+      terrainDmf: `assets/maps/${MAP_NAME}/meshes/chunks/chunk_${x}_${y}_${z}_0.dmf`,
+      terrainModel: `assets/maps/${MAP_NAME}/models/chunks/chunk_${x}_${y}_${z}_0.json`,
     };
   }
 
@@ -767,7 +769,7 @@
   async function buildChunkBrowseList() {
     const list = document.getElementById("chunkBrowseList");
     list.innerHTML = "";
-    const res = await apiLs("assetsraw/chunks");
+    const res = await apiLs(`assetsraw/maps/${MAP_NAME}/chunks`);
     for(const entry of res.entries || []) {
       const m = entry.name.match(/^chunk_(-?\d+)_(-?\d+)_(-?\d+)\.json$/);
       if(!m) continue;