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>

init.lua

@@ -9,6 +9,7 @@ local command    = require("lib-core.command")
 
 local demo_map = maps.load("maps/demo.map.json")
 maps.set_current(demo_map)
+maps.load_textures(engine.module.asset_aliases())
 
 local m_size = maps.size()
 local t_size = maps.tile_size()