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/Server/StatsLogger.js

@@ -0,0 +1,38 @@
+define([
+    "fs",
+    "path"
+],
+
+function (fs, path) {
+
+    "use strict";
+
+    var runsDir = path.join(process.cwd(), "runs");
+    if (!fs.existsSync(runsDir)) {
+        fs.mkdirSync(runsDir, { recursive: true });
+    }
+
+    var runFile = path.join(
+        runsDir,
+        new Date().toISOString().replace(/[:.]/g, "-") + ".jsonl"
+    );
+    var location = process.env.POC_LOCATION || "unknown";
+
+    fs.writeFileSync(runFile, JSON.stringify({
+        type: "run_start",
+        t: Date.now(),
+        location: location,
+        pid: process.pid
+    }) + "\n");
+    console.log("[stats] logging to " + runFile);
+
+    return {
+        log: function (obj) {
+            obj.t = obj.t || Date.now();
+            obj.location = obj.location || location;
+            fs.appendFile(runFile, JSON.stringify(obj) + "\n", function () {});
+        },
+        getFile: function () { return runFile; },
+        getLocation: function () { return location; }
+    };
+});