Net: NetTransport seam + Steam transport (Workstream C.1)

The wire moves behind NetTransport (L4NETTRANSPORT): L4NET.CPP taken
from RP412 post-seam -- all ~24 Winsock call sites route through
NetTransport_Get() -- with BT's 3 BT_NET_TRACE blocks re-sited onto
their code anchors (they read message/packet metadata, not sockets, so
no collision). Default WinsockNetTransport = the arcade/LAN TCP wire.

SteamNetTransport (L4STEAMTRANSPORT, ISteamNetworkingSockets + FakeIP/
SDR) compiles under option(BT412_STEAM) (default OFF); Steamworks SDK
1.64 vendored at extern/steamworks_sdk_164. steam_appid.txt gitignored
(Spacewar 480 by hand until a real AppID). Ported gConsoleLossEndsMission
from RP412's APPMGR (default False = arcade re-listen).

Verified: default TCP build passes full loopback MP through the seam
(console -> egg msgID-3 chunks -> mesh complete -> both instances tick,
net-tx/net-rx traces fire through NetTransport_Get()); BT412_STEAM=ON
compiles + links against the SDK + boots solo. Live Steam session
deferred to Phase 6. (Phase 4 of docs/BT412-ROADMAP.md)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

engine/MUNGA/APPMGR.cpp

@@ -5,6 +5,13 @@
 
 HWND ghWnd = 0;
 
+// BT412: losing the console mid-mission ends it (lobby-member / marshaled
+// races -- the in-process console owns the clock, so a dropped console
+// would otherwise count up forever).  Arcade -net pods leave it False and
+// re-listen for their console to return, exactly as always.  Set by the
+// front end / lobby member path (Phase 5/6).
+Logical gConsoleLossEndsMission = False;
+
 ApplicationManager* ApplicationManager::CurrentAppManager = NULL;
 
 ApplicationManager::ApplicationManager(HINSTANCE hInstance, HWND hWnd, Scalar frame_rate) : Node(ApplicationManagerClassID), runningApplications(this)

engine/MUNGA/APPMGR.h

@@ -4,6 +4,11 @@
 
 extern HWND ghWnd;
 
+// BT412: when True, losing the console mid-race ends the mission (the
+// marshaled/lobby path where the in-process console owns the clock);
+// False = the arcade -net pod re-listens for its console.  See APPMGR.cpp.
+extern Logical gConsoleLossEndsMission;
+
 class ApplicationManager : public Node
 {
 public: