fix(code-demo): graph mode buttons now actually filter the symbol graph

- Replace svgEl.className= with svgEl.setAttribute('class',...): SVG elements expose className as a read-only SVGAnimatedString, so direct assignment silently fails and the graph-dead / graph-impact CSS classes never applied - Impact mode: auto-select Router.dispatch on entry so the impact subgraph is visible immediately rather than dimming everything with no feedback