feat(selfbot): broadcast desktop audio + smart subtitles in the browse scenario

Two broadcast-experience improvements:

- Audio: the Go-Live stream was video-only. Capture the desktop sound (the
  default PipeWire/Pulse sink monitor, @DEFAULT_MONITOR@) as a second ffmpeg
  input and mux AAC into the mpegts; the library re-encodes it to Opus for
  Discord. Controlled by STREAM_AUDIO / STREAM_AUDIO_SOURCE (default on). ffmpeg
  inherits XDG_RUNTIME_DIR to reach the pulse socket. Verified: the streamer now
  reports "Found audio stream" and the monitor carries Chrome audio (~-11 dB).
- Subtitles: in the browse scenario, default captions OFF, but auto-enable a
  Korean track when the video offers one (getOption captions tracklist ->
  setOption / unloadModule).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

bot/scripts/stream-test/stream-hold.ts

@@ -18,6 +18,8 @@ const config = {
   vncFramerate: parseInt(process.env.VNC_FRAMERATE ?? "60", 10),
   vncBitrateKbps: parseInt(process.env.VNC_BITRATE_KBPS ?? "8000", 10),
   streamHw: (process.env.STREAM_HW ?? "1") !== "0",
+  streamAudio: (process.env.STREAM_AUDIO ?? "1") !== "0",
+  streamAudioSource: process.env.STREAM_AUDIO_SOURCE ?? "@DEFAULT_MONITOR@",
 } as any;
 
 const guildId = process.env.DISCORD_GUILD_ID;