MasterBus: stereo master fader, mute, level meters in PiPedal theme
(purple accent per mixer-engine branch reference).
BusStrip: added per-channel routing selector (ch1Route..ch8Route)
for selecting output bus target (Main, Bus 2-8).
MixerPage: integrated MasterBus with levels derived from bus
master data. Local state for volume/mute until backend exposes
master output.
