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>
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