Unit 1 of the sound-bar refactor. The model stores linear gain; the dock shows decibels, because
"−6 dB" is readable and "0.50" is not. `Helpers/GainDb` owns the mapping:
- 0 dB = unity (the old 1.0), range −60 dB (true silence, exactly 0 gain) … +6 dB.
- Labels for the fader readout: "−∞", "-6.0", "+1.5".
- `MonoFold(L,R)` = (L+R)/2, the accessible downmix: hard-panned content becomes audible on both
sides at half amplitude instead of absent from one. Creator context: profoundly deaf on the
right, so single-sided audio is inaudible, not merely annoying.
Guard: GainDbTests pins unity, the −6 dB half, the exact-zero floor, the +6 dB ceiling, a
round-trip across the range, the labels, and the fold arithmetic (a right-only 0.5 arrives on the
left at 0.25).
