Build the building editor

A Sims-style editor for a held plot (#173, docs/building-editor.md): draw the foundation, walls, floors, openings, stairs, storeys, roofs and furniture; see the bill, the salvage from what the plan tears down and what is still due (one small cost line; the plan is drawn as it will stand, no tints); commit; deliver materials; the shell is raised at once; furniture comes from one furniture stock per character for all towns, filled by crafting and buying.

Steps (§ 9 of the doc):

1. Blueprint core in uo-buildings (data model, from_building, pieces, rooms, checks, diff, bill/salvage/due, stable piece slots) with tests.
2. The editor in the client, offline (--build-editor): tools, ghosts, wall views, ledger, problems, undo, local save, templates. Built (#175); left for later: Furnish room, choosing the front door, placing library blueprints by the cursor. Interface (#179): a build-mode screen with pixel-art tool icons, pictures of every piece made from the baked sprites, the piece in hand between turn buttons, level arrows and four views.
3. Online: draft storage, commit, salvage, the instant raise, old #129 houses converted on load. Done (#176): records, commands, the raise, the house window's Plan and ledger, the builder bot and MCP tools; the furniture stock is a seam (backpack) until step 4 lands.
4. The furniture stock (crafting and buying into it, counts in the catalog, waiting pieces filled as the stock grows, #129 furniture items moved into stocks). Built (#174): the stock, crafting/buying/selling, #129 items swept into it, the wire, SDK, MCP and the backpack window; left for steps 2–3: the editor catalog's counts (EconMirror::furniture) and filling waiting pieces from the owner's stock.

Done when a tester rebuilds their house into a plan of their own, delivers the difference and sees it raised, and the furniture they craft or buy appears on its planned spot. Exchange buy requests for missing furniture are later (they need an order book).