Add synthetic array (waveform) DSP support + UX improvements

Adds full array/waveform support through the synthetic DSP engine: a
dsp.Sample value model (scalar or []float64), array ops (index, slice,
sum, mean, min, max, length, fft) with an in-tree radix-2 FFT, and static
type propagation (OpOutputType) that the editor mirrors to colour wires by
data type and flag invalid wirings. Stateful filters and lua stay
scalar-only. Adds a waveform plot mode (x-vs-index trace).

Also: errored-node hover reasons, S/N add-signal/add-node HUD shortcuts in
the synthetic editor, and view-mode widgets that blend with the canvas
background (chrome kept in edit mode).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/Canvas.tsx

@@ -155,7 +155,7 @@ export default function Canvas({ iface, onNavigate, timeRange }: Props) {
 
   return (
     <div class="canvas-container">
-      <div class="canvas-area" style={`width:${iface.w}px;height:${iface.h}px;`}>
+      <div class="canvas-area canvas-view-bare" style={`width:${iface.w}px;height:${iface.h}px;`}>
         {iface.widgets.map(widget => {
           const Comp = COMPONENTS[widget.type];
           const inner = Comp