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.

bot/src/stream/selfbot.ts

@@ -131,7 +131,12 @@ export class SelfbotStreamer implements ScreenStreamer {
       // RTX 5050); otherwise it falls back to software x264.
       const hw = this.config.streamHw;
       const kbps = this.config.vncBitrateKbps;
-      const maxKbps = Math.round(kbps * 1.5);
+      // The library advertises a hardcoded max_bitrate of 10 Mbps to Discord
+      // (BaseMediaConnection: `max_bitrate: 10000 * 1000`). If the encoder bursts
+      // above that negotiated ceiling, WebRTC congestion control drops packets
+      // and the viewer sees stutter. Cap -maxrate at 10 Mbps to stay within it.
+      const LIB_MAX_BITRATE_KBPS = 10000;
+      const maxKbps = Math.min(Math.round(kbps * 1.5), LIB_MAX_BITRATE_KBPS);
       const captureCodecArgs = hw
         ? ["-c:v", "h264_nvenc", "-preset", "p4", "-tune", "ll", "-forced-idr", "1"]
         : ["-c:v", "libx264", "-preset", "ultrafast", "-tune", "zerolatency"];