Footswitch bugfixes

[sastraxi]

Reworks the LCD footswitch strip (on v3) so stays in sync with MOD-UI and behaves consistently whether it is bound to a bypass or to an arbitrary parameter.

What changed

1. A footswitch bound to a parameter other than :bypass now derives its on/off state directly from the parameter value, without the bypass inversion. This means toggle parameters like a "Solo" switch light up exactly when MOD-UI says they are on, rather than silently staying constant.
2. Plugin.set_param_value is introduced as the single path for updating a parameter's cached value and pushing that value to every control bound to it. Mod now routes both param_set and connect-dump updates through this helper, so bound footswitches (and future controls) stay in sync without duplicating the update logic.
3. ShroudedPanel gives modal overlays a translucent dark backing that dims whatever is underneath, without requiring the underlying panel to redraw. It integrates with PanelStack so non-input panels can be shown without stealing focus.