Guard stream_audio with _is_audio_ready() so that PCM frames are
dropped (not crashed on) when pymumble recreates SoundOutput with
encoder=None during reconnect. The ffmpeg pipeline stays alive,
position tracking remains accurate, and audio feeding resumes once
the codec is negotiated. Listeners hear brief silence instead of
a 30+ second restart with URL re-resolution.
Also adds chat messages to _auto_resume so users see what the bot
intends ("Resuming 'X' at M:SS in a moment" / "...aborted").
ab924444de