graphTemplateHandler lowers a {nodes,edges} payload to SVG: a deterministic
circular layout of labelled <circle> nodes with <line> edges, self-contained
(own light backdrop + content colors, like a d2 card) and painted by the
shared renderer. Display-only (D-78); decoupled from the interactive graph
pane (T-323) since a static card only needs a layout->SVG. Honest error on
a duplicate id or an edge to an unknown node.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>