core v0.5.0

Pre-game lobby, co-op settings panel, HUD drawing for games, clean exit (new features, so a minor release). No
breaking changes: game repos update without code changes; every new interface call is optional. The wire protocol
changes (kProtocolVersion 7, the lobby messages), so players need the same build anyway.

• Lobby (net/lobby.h, docs/protocol.md "Lobby"): guests ready up, the host picks a mode from the game's list and
  starts it for everyone; a late joiner sees the game in progress and can follow it. The dll attaches it to every
  session and writes mc_state / mc_mode into the Steam lobby data (SteamLobby::SetData). New test net_lobby.
• Co-op UI: a LOBBY panel (players with character and READY / NOT READY / HOST, the game's character rows, READY or
  MODE + START, JOIN GAME while the host plays, INVITE FRIENDS, SETTINGS, LEAVE); it opens by itself after joining a
  host while not in the world (once GameModule::LobbyCanOpen()), closes on every screen when the game starts and
  when the session ends. A CO-OP SETTINGS panel (NAME with a text box, DEBUG OVERLAY, the game's rows; saved with
  Config::Set), also reachable from the pause CO-OP panel. New MenuCommand::Lobby / Settings.
• GameModule (all optional): SettingsOptions / SettingsOptionStep, LobbyModes / StartMode, InWorld /
  CurrentMode / LobbyCanOpen, PlayerCharacter, DrawHud (game geometry under the panels, e.g. name tags).
• Clean exit: ExitProcess and TerminateProcess(self) are hooked; the session is left cleanly before the process
  ends, so the others see "host left" / " left the game" at once instead of after the 10 s timeout. Banners:
  HOST LEFT for guests, " lost the connection" for a timeout. UDP sends its Disconnect 4 times (was 2).
• Overlay: holds no backbuffer reference between frames; releases everything on device removed/reset and at exit;
  logs the Present / ResizeBuffers threads. Mod::DebugOverlay / SetDebugOverlay, Coop::SetName.