SVG rendering

The SVG renderer walks a fully laid-out `IrGraph` and emits an SVG document as a string. SVG is generated directly as XML text using a small writer helper for indentation and escaping, with no external SVG library.

The rendering order is: compute the total bounding box from all node positions, edge points, and cluster boxes, adding margin from the `pad` attribute (default 4pt); emit the `` root with a `viewBox`; render clusters as `` elements with a background rectangle and label; render edges as `` elements (polyline or cubic Bezier) with inline arrowhead polygons; render nodes as `` elements containing the shape and a `` label. Multi-line labels using Graphviz's `\n`, `\l`, and `\r` escape sequences are split into multiple `` elements with explicit `dy` offsets. The `color`, `fillcolor`, `fontcolor`, `fontsize`, `fontname`, `bgcolor`, `margin`, and `pad` attributes are all resolved during rendering.

References

• SVG specification (W3C)
• SVG viewBox (MDN)
• SVG text element (MDN)
• SVG tspan element (MDN)
• Graphviz output formats

Start with the IrGraph representation and the SVG renderer's writer entry point described in the issue. Confirm how layout positions, edge points, cluster boxes, and rendering attributes are exposed. Done means producing valid SVG with the stated viewBox, clusters, edges, nodes, escaped multiline labels, resolved styling, and padding.