Prepare consistent Chunk sizes and fresh Fast Fetch integration

utils/chunk-size-benchmark/datachannel-metrics.cjs

@@ -0,0 +1,23 @@
+const { writeFileSync } = require('node:fs');
+const { RTCDataChannel } = require('werift');
+
+const output = process.env.LIVESYNC_BENCH_DATACHANNEL_JSON;
+if (!output) throw new Error('LIVESYNC_BENCH_DATACHANNEL_JSON is required');
+const channels = new Set();
+const originalSend = RTCDataChannel.prototype.send;
+RTCDataChannel.prototype.send = function (...args) {
+    channels.add(this);
+    return originalSend.apply(this, args);
+};
+process.once('exit', () => {
+    const rows = [...channels].map((channel) => ({ label: channel.label, payloadBytesSent: channel.bytesSent, messagesSent: channel.messagesSent }));
+    const cpu = process.cpuUsage();
+    writeFileSync(output, JSON.stringify({
+        source: 'werift 0.24.4 RTCDataChannel counters; observed channels through a benchmark-only preload',
+        scope: 'Application DataChannel payload; includes application retransmissions, excludes SCTP/DTLS/UDP/IP overhead, transport retransmissions, and relay signalling',
+        payloadBytesSent: rows.reduce((sum, row) => sum + row.payloadBytesSent, 0),
+        messagesSent: rows.reduce((sum, row) => sum + row.messagesSent, 0),
+        processUsage: { userCpuMs: cpu.user / 1000, systemCpuMs: cpu.system / 1000, maxResidentSetKiB: process.resourceUsage().maxRSS },
+        channels: rows,
+    }, null, 2));
+});