fix: cap selfbot stream -maxrate at lib's 10 Mbps ceiling; add stream-test tooling

- selfbot.ts: the @dank074 lib advertises a hardcoded max_bitrate of 10 Mbps to
  Discord (BaseMediaConnection: `max_bitrate: 10000 * 1000`). Our encoder used
  -maxrate = 1.5x target (12 Mbps at 8 Mbps target), so high-motion bursts
  exceeded the negotiated ceiling and WebRTC dropped packets (viewer stutter).
  Cap -maxrate at 10 Mbps.
- Add bot/scripts/stream-test/: env-driven stream-hold.ts (persistent Go-Live
  holder), human.mjs (real xdotool mouse/keyboard + char-by-char typing), and
  scenario.mjs (YouTube/Naver browse). Channel/guild/video are env-parametrised.
- .env.example: document DISCORD_VOICE_CHANNEL_ID for the stream-test scripts.

.env.example

@@ -11,6 +11,8 @@ DISCORD_BOT_TOKEN=
 DISCORD_APP_ID=
 # The (single) server this bot serves. Guild-scoped commands appear instantly.
 DISCORD_GUILD_ID=
+# Voice channel used by the stream-test scripts (bot/scripts/stream-test).
+DISCORD_VOICE_CHANNEL_ID=
 
 # ---------------------------------------------------------------------------
 # Brain bridge (Python service in bridge/) — STT + reply engine + TTS