Sell town houses with their lots

Most town buildings become lots for sale with their house standing (#173, docs/building-editor.md § 1.2): NPCs need only a few. The town keeps the bank with the exchange, the temple, the barracks and one inn; every other building's plot is a lot.

• Every baked building gets its plot in world.json (today only the 10–20 % free plots are listed) and a flag whether the town keeps it.
• Holding such a plot gives the house: its building is regenerated from the town plan's spec and becomes the lot's first built plan; the baked statics of that building are hidden by a server patch.
• Decide the price beyond the plot's stake (default: the house's materials in coin to the treasury) and where shopkeepers and smiths of sold buildings go (default: market stalls).

Done (#177): lots with their standing house in world.json, price = stake + materials in coin to the treasury, a deed to move in with, the house record hides the baked building on the server and every client, moving out leaves it vacant for the next buyer, shopkeepers and smiths of sold buildings at stalls around the exchange. Open: a town house's record gets its built plan once HouseView.plan lands (server-plans), generator furniture is not kept.