feat: smooth volume ramping over 200ms in audio streaming

Volume changes now ramp linearly per-sample via _scale_pcm_ramp instead
of jumping abruptly. Each frame steps _cur_vol toward target by at most
0.1, giving ~200ms for a full 0-to-1 sweep. Fast path unchanged when
volume is stable.

docs/USAGE.md

@@ -1565,7 +1565,7 @@ and voice transmission.
 - `libopus` -- Opus codec (used by pymumble/opuslib)
 
 ```
-!play <url>          Play audio or add to queue
+!play <url|playlist> Play audio or add to queue (playlists expanded)
 !stop                Stop playback, clear queue
 !skip                Skip current track
 !queue               Show queue
@@ -1576,8 +1576,11 @@ and voice transmission.
 ```
 
 - Queue holds up to 50 tracks
-- Volume takes effect immediately during playback (default: 50%)
-- Title resolved via `yt-dlp --get-title` before playback
+- Playlists are expanded into individual tracks; excess tracks are
+  truncated at the queue limit
+- Volume changes ramp smoothly over ~200ms (no abrupt jumps)
+- Default volume: 50%
+- Titles resolved via `yt-dlp --flat-playlist` before playback
 - Audio pipeline: `yt-dlp | ffmpeg` subprocess, PCM fed to pymumble
 - Commands are Mumble-only; `!play` on other adapters replies with an error,
   other music commands silently no-op