perf(golive): ffmpeg-spawn demuxer (no node-av) + E2E pipeline tests

Phase 2 — replace the 114MB node-av binary with a plain ffmpeg spawn:

Demuxer.ts: spool stream input to temp file → probe via ffmpeg stderr
(ffmpeg-headless ships NO ffprobe — parse 'Stream #0:0: Video: h264...
640x360, 30 fps' from -loglevel info) → ffmpeg -c copy -f h264 pipe:1
→ NAL-split frames. Falls back to h264 defaults when probe fails.

prepareStream.ts: resolve ffmpeg from FFMPEG_PATH env → Nix store
ffmpeg-headless (hash-prefixed entry!) → PATH; split encoder option
strings ('-forced-idr 1' → two argv) — fluent-ffmpeg used to split
automatically, spawn does not.

E2E tests (tsx, need LD_LIBRARY_PATH=/tmp/ldc-build):
- golive-demux-e2e.ts: real H264 file → 33 NAL frames + dims from probe
- golive-pipeline-e2e.ts: prepareStream → demux → 82 frames
- golive-videostream-e2e.ts: local peer pair → demux → VideoStream →
  native setPacketizer/sendFrame/addTimestamp → 33 frames sent connected

Pitfalls captured: setPacketizer before negotiation breaks createOffer
('No DataChannel or Track to negotiate'); track methods are read-only
(no monkeypatching); both peers must declare audio+video tracks or
answer hangs; state() returns 'closed' after close() — snapshot first.

services/discord-gateway/tests/golive-demux-e2e.ts

@@ -0,0 +1,40 @@
+// Phase 2 E2E: Demuxer on a real ffmpeg-generated H264 file.
+// Run: npx tsx tests/golive-demux-e2e.ts
+
+import { createReadStream } from "node:fs";
+import { demux } from "../src/goLive/Demuxer.js";
+
+const input = process.argv[2] ?? "/tmp/sample.h264";
+const { video, close } = await demux(createReadStream(input), {
+  format: "h264",
+});
+
+console.log(
+  "video:",
+  JSON.stringify({
+    codecName: video.codecName,
+    width: video.width,
+    height: video.height,
+    duration: video.duration,
+    fps: Math.round(video.framerate_num / video.framerate_den),
+  }),
+);
+
+let count = 0;
+let keyframes = 0;
+let bytes = 0;
+video.stream.on("data", (frame: { data: Buffer; keyframe: boolean }) => {
+  count++;
+  bytes += frame.data.length;
+  if (frame.keyframe) keyframes++;
+});
+video.stream.on("end", () => {
+  console.log(`frames: ${count} (${keyframes} keyframes), ${bytes} bytes`);
+  close();
+  process.exit(0);
+});
+video.stream.on("error", (e: unknown) => {
+  console.error("stream error:", e);
+  close();
+  process.exit(1);
+});