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/scenario.mjs

@@ -43,6 +43,23 @@ async function browserFullscreen(on) {
   } catch { /* best-effort */ }
 }
 
+// Subtitles: OFF by default, but turn ON Korean when a Korean track exists.
+async function applyCaptions() {
+  await read(() => { try { document.getElementById('movie_player')?.loadModule?.('captions'); } catch {} });
+  await sleep(800);
+  return read(() => {
+    const pl = document.getElementById('movie_player');
+    if (!pl || !pl.getOption) return 'no-player';
+    let tracks = [];
+    try { tracks = pl.getOption('captions', 'tracklist') || []; } catch {}
+    const ko = tracks.find((t) => /^ko/i.test(t.languageCode || ''));
+    if (ko) { try { pl.setOption('captions', 'track', { languageCode: ko.languageCode }); } catch {} return 'ko-on'; }
+    try { pl.setOption('captions', 'track', {}); } catch {}
+    try { pl.unloadModule('captions'); } catch {}
+    return 'off';
+  });
+}
+
 // 1) open YouTube by typing the URL in the address bar
 await navigateOmnibox('https://www.youtube.com'); await sleep(3000);
 
@@ -84,6 +101,7 @@ async function setQuality1080() {
   return null;
 }
 console.log('QUALITY', await setQuality1080());
+console.log('CAPTIONS', await applyCaptions());
 
 // 6) turn off autoplay with a real click if it is on
 const auto = page.locator('.ytp-autonav-toggle-button');