When the Direct arm moved from a concrete AlsaDirectStream to
Arc<dyn DirectSink>, the set_hardware_volume call kept its
cfg(target_os = "linux") gate, so on every other platform the setting
silently did nothing.
Added as a second arm rather than by removing the Linux one, per the
protected-code rule. Linux keeps its verbatim block and its behaviour;
a Windows sink can now answer for itself, and one with no hardware volume
returns Err and says so instead of the call never happening. macOS and the
remaining targets keep their previous silence, since CoreAudio has its own
path through set_coreaudio_hardware_volume.
Latent rather than live: the row is visible only on ALSA.
cc
vicrodh
ff6f40f9dd