D1 phase 2+3: client TCP relay mode (L4NET) + 2-node verification

The pod can now run entirely over the relay: one outbound TCP connection
carries both the console protocol (egg/launch) and all game traffic
(envelope-multiplexed), so internet play works behind NAT with no port
forwarding.  All new code is behind 'if (relayMode)' (BT_RELAY unset => mesh
mode byte-for-byte untouched; verified un-regressed).

Env gates (parsed once in the ctor, after WSAStartup so gethostbyname works):
- BT_RELAY=<host>:<consolePort>  (host may be a DNS name; game port = +1)
- BT_SELF=<exact [pilots] entry> (NIC matching can't identify us across NAT)

L4NET changes:
- CreateConsoleHost: relay branch dials OUT to the relay console port
  (bounded retry; console-less continue on a re-dial) instead of listening.
- StartConnecting: relay self-match by BT_SELF string; peers become VIRTUAL
  hosts (INVALID_SOCKET, NoNetworkConnectionStatus) flipped online by the
  relay's PEER_UP; then ConnectRelayGame dials the game port + HELLOs.  HostID
  assignment / remoteHostCount / the connection gate / app ladder unchanged.
- CheckRelay (new): the receive seam -- drains {route,length} envelopes,
  synthesizes HostConnected/Disconnected from PEER_UP/DOWN (same messages the
  mesh accept path routes), returns game frames envelope-stripped (they
  self-identify via NetworkPacketHeader.fromHost; consumers route by payload).
- Send: game-host traffic -> relay unicast envelope (console host excluded --
  its legacy protocol is relay-TERMINATED, not routed).
- ExclusiveBroadcast: build ONCE, send ONCE with the broadcast route -- the
  relay fans out, killing the mesh's (N-1)x upload duplication.
- RelaySendAll (new): partial-send-safe transmit (required on the multiplexed
  socket -- a partial write would desync framing for all peers).
- CheckBuffers: polls CheckRelay first; skips recv on virtual game hosts.
- RelayGameDown: relay-loss synthesizes all-peers-disconnected (match
  continues peer-less; pod never exits mid-match).
- Mode(): now STORES the reliable/unreliable mode (was ignored) for the UDP
  phase's authentic mode+flag routing.

Verified 2-node localhost relay (MP_RELAY.EGG, tagged [pilots]):
- both pods reach 'All connections completed!' via PEER_UP, then
  RunningMission; both mechs' MakeMessages cross the relay (paint x2 each) and
  bidirectional 148-byte update records flow (net-tx/net-rx traces; relay
  stats tcp rx==tx, registered [2,3]); cockpit/HUD render; no crashes.
- driving one node transmits pose/damage/death frames through the relay.
- mesh smoke (no BT_RELAY): 2-node session still forms + simulates unchanged.

Plan: ~/.claude/plans/partitioned-snuggling-piglet.md.  Next: UDP channel.

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

engine/MUNGA_L4/L4NET.H

@@ -327,6 +327,30 @@ private:
 			DroppedMessageHostSocket *dropped_message_host_socket
 		);
 
+	//
+	// D1 RELAY MODE (BT_RELAY=host:port + BT_SELF=<[pilots] entry>): outbound-
+	// only topology -- the pod dials the relay (extended btconsole.py) for BOTH
+	// the console protocol (egg/launch; legacy raw frames, direction flipped)
+	// and the game traffic (envelope-framed, one TCP connection multiplexing
+	// every peer; UDP for the unreliable channel in a later phase).  BT_RELAY
+	// unset => relayMode False => every relay branch is dead and the classic
+	// mesh is untouched.
+	//
+	SOCKET
+		RelayDialTcp(const SOCKADDR_IN &endpoint, int timeout_seconds);
+	Logical
+		RelaySendAll(SOCKET sock, const char *buffer, int length);
+	NetworkPacket *
+		BuildRelayPacket(Message *message, ClientID client, int *packet_size);
+	Logical
+		RelaySendFrame(int route, const NetworkPacket *packet, int packet_size);
+	void
+		ConnectRelayGame(HostID local_host_ID);
+	void
+		RelayGameDown(const char *why);
+	Logical
+		CheckRelay(NetworkPacket *network_packet);
+
 //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 // Private Data
 //
@@ -353,6 +377,23 @@ private:
 	addrinfo* thisHost;
 	SOCKET gameListenerSocket;
 	SOCKET consoleListenerSocket;
+
+	// D1 relay mode state (see the private-methods block above).  All inert
+	// unless BT_RELAY is set.
+	Logical relayMode;
+	char relaySelf[64];					// BT_SELF -- our [pilots] entry string
+	SOCKADDR_IN relayConsoleAddress;	// relay console endpoint (BT_RELAY port)
+	SOCKADDR_IN relayGameAddress;		// relay game endpoint (port + 1, tcp+udp)
+	SOCKET relayGameSocket;				// the ONE multiplexed game connection
+	SOCKET relayUdpSocket;				// unreliable channel (later phase)
+	char relayPad[16384];				// relay-connection reassembly buffer
+	int relayPadTail;
+	Logical relayConsoleDialedOnce;		// first dial gets the long timeout
+	Logical udpUp;						// UDP HELLO acked by the relay
+	unsigned long lastUdpKeepaliveTick;
+	unsigned long udpTxSeq;
+	unsigned long udpRxSeq[64];			// per-fromHost last-seen sequence
+	NetworkMode currentNetworkMode;		// Reliable/Unreliable (Mode() stores)
 };
 
 //~~~~~~~~~~~~~~~~~~~~~~ L4NetworkManager inlines ~~~~~~~~~~~~~~~~~~~~~~~~~~