World props: rocks, cliffs and ores

Model and bake the 107 sprites of group stone of the world-props order (assets/requests/world-props/stone.md, brief assets/requests/world-props.md) on their own branch. Done when every sprite of the list is in assets/manifest.d/world_stone.toml, baked and reviewed on a gallery sheet.

Done (review): all 107 sprites are in assets/manifest.d/world_stone.toml, baked (182 frames, every one on the texel lattice) and reviewed on gallery sheets; cliff runs, corners and ends were checked in composites. Cliff placement (for dress.rs): _x / _y on the north / west edge with the mountain behind; inner = both on one tile (mountain north and west); corner and end go on the tile at the north-west corner of which an x run arriving from the west and a y run arriving from the north stop (east of the last x piece, south of the last y piece). Runs that stop the other way (x run ending westwards, y run ending northwards) have no piece yet: they end against the terrain, or need mirrored pieces later.