fix(goLive): demux access-unit grouping + correct RTP timestamps (black tile)

Demuxer emitted each AnnexB NAL as its own WebRTC frame (SPS/PPS/SEI
separate from slices) with a near-zero timestamp delta (duration=1 in a
1/90000 timebase → RTP +1/frame instead of +3000 @30fps). Discord's H264
receiver never receives a complete decodable access unit → black GoLive
tile despite frames flowing.

- Group NALs into access units: buffer param-set/SEI NALs, flush one
  frame per slice with preceding parameter sets (AnnexB start codes kept
  so the H264RtpPacketizer finds NAL boundaries).
- Timestamp each frame at the video frame rate: duration=1, timeBase
  1/fps → BaseMediaStream frametime=1000/fps ms → RTP +clockRate/fps
  (3000 @ 30fps/90kHz) and correct pacing.
- Thread explicit frameRate from playStream options (raw H264 has no
  timing info; ffmpeg guesses 25fps on stderr).
- Strengthen golive-demux-live-e2e: validates every frame has a slice,
  no bare param-set frames, keyframes carry SPS/PPS, timeBase 1/30.

services/discord-gateway/src/goLive/prepareStream.ts

@@ -266,6 +266,8 @@ export async function playStream(
 
   const { video, close: demuxClose } = await demux(prepared.output, {
     format: options.format ?? "nut",
+    frameRate:
+      typeof options.frameRate === "number" ? options.frameRate : undefined,
   });
   console.log(
     `[goLive:playStream] demux done codec=${video?.codecName ?? "?"} ${video?.width ?? 0}x${video?.height ?? 0} fps=${video ? video.framerate_num / video.framerate_den || 30 : 30}`,