Model diagonal walls, low walls, arrow slits and crenellations

The building generator places wall_<m>_diag_a/_b[_window], _low_{x,y,corner,post}, _slit_{x,y} and _crenel_{x,y,corner,post} for every full wall set; the client draws placeholders until they are baked. Contract and sizes: assets/requests/wall-pieces.md. Done when every piece is in assets/manifest.d/walls.toml, baked on the texel lattice, and uo-buildings reports no missing wall sprites.

Delivered on feat/wall-pieces (#102): all 14 pieces for the nine sets are in the manifest and baked on the texel lattice; the village shows them in a mason's yard.