Route gameCommand traffic through WebRTC unreliable DataChannel

Socket.IO (TCP) holds back later packets while it retransmits a lost
one, which stalls worldUpdate delivery on lossy long-distance links —
exactly the pattern game state suffers worst from. WebRTC DataChannels
in unreliable mode (ordered:false, maxRetransmits:0) drop late packets
instead of queueing them, which is what we want for high-frequency
state sync.

Adds a per-user WebRTCTransport on top of the existing Socket.IO
connection. Socket.IO stays in charge of bootstrap, signaling
(SDP/ICE exchange), and control messages — only gameCommand payloads
get routed onto the unreliable channel once it's open. If WebRTC
fails to negotiate, gameCommand transparently falls back to
Socket.IO, so the game keeps working unchanged.

A new StatsLogger writes per-session JSONL events (session_start,
webrtc_ready with negotiation time, per-second stats with transport,
RTT samples, recv/send rates, seq gaps) so we can compare real-world
runs (e.g. Germany server <-> Korea client) instead of guessing.
URL flag ?webrtc=0 forces fallback for A/B testing.

scripts/webrtc-browser-test.js spins up a headless Chromium against
a freshly-started server and asserts the unreliable channel opens
and gameCommand traffic actually rides it.

app/Game/Config/Settings.js

@@ -83,6 +83,8 @@ function () {
         NETWORK_LOG_INCOMING: false,
         NETWORK_LOG_OUTGOING: false,
         NETWORK_LOG_FILTER: ["ping", "pong", "worldUpdate", "lookAt"],
+        USE_WEBRTC: true,
+        WEBRTC_STUN_SERVERS: ["stun:stun.l.google.com:19302"],
 
         // CHANNEL
         CHANNEL_MAX_USERS: 20,