Net: Steam lobby (btl4lobby) + launch-mode wiring (Workstream C.2)

Port RP412's RPL4LOBBY to BT: an ISteamMatchmaking room stands in for the
arcade Site-Management screen. Compiled both gates -- stubs without
BT412_STEAM, the full room under it (both configs build + link clean).

Lobby (game/reconstructed/btl4lobby.*):
- Room screen (green-on-black, like the menu), owner = console.
- Member data: FakeIP + fake console/game ports + persona + mech/color/badge
  (ip/cp/gp/nm/vh/cl/bd keys).
- Nonced "go" launch roster -> SteamNetTransport_RegisterPeer for every peer.
- Push/PullRaceResults over lobby data rebuild the shared score sheet.

Front end (btl4fe.*): HOST/JOIN buttons when BTLobby_Available() (Steam
transport up); the menu loop exits on a steam action and BTFrontEnd_Run
routes the lobby outcome -- host builds the egg with every member as a
[pilots] mesh entry (BTFrontEnd_SetHostedPilots), member returns launch
mode 2. Results screen prefers the marshal's collated result names.

Marshal (btl4console.*): add GetResultName / ClearResults / InjectResult so
the lobby owner can refill the sheet from the collated wire scores; Result
gains a name field.

WinMain (btl4main.cpp): install the Steam transport on BT412STEAM env
(before the front end, so the lobby is offered); branch on
BTFrontEnd_LastLaunchMode() -- host owns the marshal clock + joins the mesh,
member enters as a network pod on :1501 (SetNetworkCommonFlatAddress +
gConsoleLossEndsMission); Push (host) / Pull (member) results, then relaunch.

CMake: btl4lobby.cpp joins bt410_l4; that lib gets BT412_STEAM + the
Steamworks include under the gate. build-steam/ gitignored.

Deferred (untestable here -- needs Steam + multiple machines): the
host->member wire egg-feed marshal (InstallNetworkRace); a member currently
waits for a console connection nothing supplies, so a live host+member race
is blocked on it. The lobby object does not survive the per-mission
relaunch. docs/STEAM-3-MACHINE-TEST.md for BT not yet authored.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

game/fwd/l4steamtransport.hpp

@@ -0,0 +1 @@
+#include "../../engine/MUNGA_L4/L4STEAMTRANSPORT.h"