canonicalD function
Canonical d attribute of an icon: a PathIcon's string verbatim, or a
NodeIcon's real cubics quantized to 4 decimals.
Flutter never needs this to draw — MorphController.path carries exact
curves. It exists for interop (handing a d to another SVG renderer) and
because it is the string the web bindings would have painted, which is what
makes cross-implementation comparison possible.
Implementation
String canonicalD(MorphIconData icon) {
if (icon is PathIcon) return icon.d;
return _canon[icon] ??= cubicsToPathD(_cubicsOf(icon));
}