Reported: pegging the TRAX fader changes nothing audible, the ladder never reaches red, and the
ladder does not scale with the fader.
Cause (two defects in the same setters):
- UserGain wrote the OUTPUT DEVICE's volume. NAudio's WaveOut ignores that, so the fader was inert.
- LocalGain (the desktop-mute follow) recomputed the sample-chain volume WITHOUT the user gain, so
every duck/mute silently discarded whatever the creator had set.
Fix: one `EffectiveVolume(user, local)` = MusicVolume × user × local, applied to the sample chain by
both setters. The meter sits AFTER that stage, so the ladder now scales with the fader and can reach
the peak band on a loud track — exactly what pegging should look like. Max user gain is +6 dB, so
"pegged" means the file at unity, not a magic boost.
Guard: MusicLevelTests pins the arithmetic (default half, pegged double, mute multiplies to zero),
that the user gain survives the mute follow, and that nothing may drive the output device's volume
again — the no-op that made this invisible.
