Switch spine-prototype demo to fa_terrain_v1 atlas

Replaces the inline color-only demo_tilemap with a dep on the
prototype-fa-starter asset lib and its baked fa_terrain_v1 atlas.
The map atlases[] alias-string is updated to fa_terrain_v1; the
packed-u32 layer-tiles keep their numeric values because the baker
assigned the expected IDs (1 for grass_field surface tile, 2 for
stone_wall_brick wall tile). Adds maps.load_textures() call in init.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

maps/demo.map.json

@@ -2,7 +2,7 @@
   "schema_version": 2,
   "id": "demo",
   "size": { "w": 16, "h": 16 },
-  "atlases": ["demo_tilemap"],
+  "atlases": ["fa_terrain_v1"],
   "layers": {
     "surface": {
       "tiles": [