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>

internal/datasource/synthetic/dsp_bridge.go

@@ -126,6 +126,30 @@ func buildNode(d NodeDef) (dsp.Node, error) {
 	case "lua":
 		return &dsp.LuaNode{Script: stringParam(p, "script"), Vars: stringSliceParam(p, "vars")}, nil
 
+	case "index":
+		return &dsp.IndexNode{I: int(floatParam(p, "i"))}, nil
+
+	case "slice":
+		return &dsp.SliceNode{Start: int(floatParam(p, "start")), End: int(floatParam(p, "end"))}, nil
+
+	case "sum":
+		return &dsp.SumNode{}, nil
+
+	case "mean":
+		return &dsp.MeanNode{}, nil
+
+	case "min":
+		return &dsp.MinNode{}, nil
+
+	case "max":
+		return &dsp.MaxNode{}, nil
+
+	case "length":
+		return &dsp.LengthNode{}, nil
+
+	case "fft":
+		return &dsp.FFTNode{}, nil
+
 	default:
 		return nil, fmt.Errorf("unknown node type %q", d.Type)
 	}