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_L4/L4NET.H

@@ -296,6 +296,11 @@ public:
 	//static MessageHandlerSet MessageHandlers;
 	static MessageHandlerSet& GetMessageHandlers();
 
+	// Load a local egg file and kick the mission cycle - the same path
+	// the single-user startup takes; used by the in-game front end /
+	// local console for the race-after-race loop.
+	void FeedLocalEgg(const char *egg_path);
+
 	void ReceiveEggFileMessageHandler(
 			ReceiveEggFileMessage *EggMessage);
 	void AcknowledgeEggFileMessageHandler(