Footswitch rewrite (more vertical space, bugfixes)

[sastraxi]

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

Happy to extract out the correctness fixes without the UI change, too.

Preview

• A/B: bound to bypass. A is on, B is bypassed.
• C/D: bound to nothing. C is on, D is off. Source of truth: pi-Stomp.

What changed

1. The old widget drew two ellipses and a label, which looked like a footswitch but took up too much space. The new FootswitchWidget renders a compact keycap outline around a centered label. Color is used as an accent: the configured plugin color when the switch is on, and a dimmed gray when it is off. Long labels are hard-truncated to the slot width so they never overflow into a neighboring switch.
2. 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.
3. 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.
4. 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.