full_mix with ducking enabled (the default) failed for a single narration
track + one music bed — the most common shape — because the ducking branch
appended an acopy[speech_dup] filter whose output pad was never consumed,
leaving the filtergraph with a dangling output that ffmpeg rejects.
For a single speech track speech_out is '[a0]' (starts with '[a'), so the
guarded append fired; the compensating pop() only removes the empty-string
case from the multi-speech branch, so the dead pad survived exactly in the
single-narration case. The speech stream is already re-derived for the final
mix via [speech_out], and ffmpeg auto-splits the reused input label, so the
duplicate is unnecessary. Multi-speech and SFX paths are unaffected.
Adds regression tests for single- and multi-narration full_mix with ducking.
Closes#265
febc9244d3