Billboard API 0.1.0 for Minecraft 26.3

Changelog

0.1.0 (not released)

• Project set up: Fabric mod for Minecraft 26.3 with Cobalt, the Lua VM, packed in under its own package name.
• Lua Box, taken over from Billboard Engine's Script Box: the program is main.lua, plus up to 8 more files of the
  types the server allows (images, .txt, .json, .csv, .lua). Other .lua files are modules for require.
  Programs read the box's files with files.list(), files.read(name) and files.lines(name).
• Everybody may press Restart and Stop; only the owner and operators may change the files.
• Settings in config/billboard_api.properties; on the first start the Script Box limits are taken over from
  Billboard Engine's billboard.properties.
• /luabox <pos> restart|stop|status for operators and command blocks.
• Worlds from Billboard Engine 1.x: Script Boxes become Lua Boxes (block, item and block entity alias from
  billboard:script_box). Each keeps its files, which move to billboard_api/boxes/, and its one program is
  renamed to main.lua. Not while Billboard Engine 1.x itself is installed.
• Lua clients: mods register machines with BillboardApi.CLIENTS (a Fabric block API lookup). Programs get
  network.list(), network.findByName(name), network.find(type), network.version() and a handle per client
  with getName(), setName(name), getType() and the client's own functions. A client may answer later with a
  CompletableFuture; the program waits for it.
• A box reaches clients that touch it, a Lua Box or a pipe joined to it. Names are kept by the library on the machine;
  a name clash gets the newcomer renamed, and every box tells. Settings network_max_clients=64,
  network_max_boxes=8, network_owner_check=true: a box drives only machines its owner placed; machines from
  before go to the owner of the first box that reaches them.
• engine still works for Script Box programs: the one Billboard Engine of the network.
• Vanilla signs and hanging signs are Lua clients, minecraft:sign.
• Data Pipe (billboard_api:data_pipe, 8 from planks, paper, planks; cardboard look like the Lua Box, wood sound): joins Lua Boxes and Lua clients
  into one network. Sneak-click an arm with an empty hand to switch that side off. network_max_pipes=512.
• Box screen: a Network button swaps the file list for the machines the box reaches, with their type; click one to
  rename it. Machines of other owners are shown greyed out.
• LuaArgs.caller(): the calling program (LuaCaller: describe(), isActive(), file(name)), so a machine can
  let one program claim it, and show files of the calling box.