D1 phase 5+6: UDP unreliable channel (restores the 1995 reliable/unreliable split)

The ~60Hz entity update records now ride UDP in relay mode, so a lost/late
datagram is dropped (dead reckoning absorbs the gap) instead of head-of-line-
blocking the reliable TCP stream -- the fix for internet rubber-banding.
Reliable traffic (make/damage/death/control/egg/launch) stays on TCP.

This RESTORES the authentic 1995 NETNUB split, it doesn't invent one:
Receiver::Message defaults messageFlags=ReliableFlag; the update path clears
it (flags=0, ENTITY.cpp:590); Mode(UnreliableMode) fires at LoadingMission
(APP.cpp:704 -- the 2007 port's Mode() was a no-op, now it STORES the mode).
Routing gate = (mode==UnreliableMode && !(flags & ReliableFlag)) -- flag AND
mode, so the map-stream creation messages (also flags=0 but flowing during the
still-Reliable CreatingMission window) ride TCP for free.

Client (L4NET):
- Mode() stores currentNetworkMode (was ignored).
- ConnectRelayUdp: UDP socket connect()ed to the relay game port; HELLO
  (outbound punch-through, relay learns our endpoint); udpUp on HELLO-ACK.
  BT_RELAY_TCP_ONLY=1 disables the channel (UDP-blocked nets degrade to the
  pure-TCP checkpoint automatically -- no ACK => everything stays on TCP).
- RelayUdpSendFrame: {route, fromHost, seq} envelope + frame, best-effort.
- RelayUdpKeepalive: ~15s NAT-hold + ~1s HELLO retry until acked.
- CheckRelayUdp: HELLO-ACK sets udpUp; per-sender seq gate drops stale/
  out-of-order datagrams; frames handed up envelope-stripped like TCP.
- Send / ExclusiveBroadcast: route unreliable-window traffic to UDP, else TCP.
- CheckBuffers polls the UDP channel first, then TCP; keepalive tick.
(The relay-side UDP forwarder + endpoint learning + --udp-drop hook landed in
phase 1.)

Verified 2-node localhost relay: during a drive the update stream flows on UDP
(relay udp tx 2->190+, TCP frozen at 53) and the peer's replicant tracks the
master to sub-2u; 15% forced drop (--udp-drop 15) stays coherent (tracked to
0.1u); TCP-only fallback confirmed (udp-known drops the TCP-only pod); no
crashes; mesh mode un-regressed.  KB: context/multiplayer.md gains the D1
RELAY MODE section.

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

engine/MUNGA_L4/L4NET.H

@@ -350,6 +350,22 @@ private:
 		RelayGameDown(const char *why);
 	Logical
 		CheckRelay(NetworkPacket *network_packet);
+	//
+	// UDP unreliable channel (restores the 1995 NETNUB reliable/unreliable
+	// split): the ~60Hz update records ride UDP so a lost/late datagram is
+	// dropped (dead reckoning absorbs the gap) instead of head-of-line-
+	// blocking the reliable stream.  Reliable traffic + control stay on TCP.
+	//
+	void
+		ConnectRelayUdp(HostID local_host_ID);
+	Logical
+		RelayUdpSendFrame(int route, const NetworkPacket *packet, int packet_size);
+	void
+		RelayUdpKeepalive(HostID local_host_ID);
+	Logical
+		RelayShouldUseUdp(const Message *message) const;
+	Logical
+		CheckRelayUdp(NetworkPacket *network_packet);
 
 //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 // Private Data
@@ -390,7 +406,9 @@ private:
 	int relayPadTail;
 	Logical relayConsoleDialedOnce;		// first dial gets the long timeout
 	Logical udpUp;						// UDP HELLO acked by the relay
+	HostID relayLocalHostID;			// our hostID (for UDP HELLO/keepalive/tx)
 	unsigned long lastUdpKeepaliveTick;
+	unsigned long lastUdpHelloTick;		// HELLO retry until acked
 	unsigned long udpTxSeq;
 	unsigned long udpRxSeq[64];			// per-fromHost last-seen sequence
 	NetworkMode currentNetworkMode;		// Reliable/Unreliable (Mode() stores)