SigmaExporter

Renders a laid-out Vista to the flat JSON shape consumed by sigma-viewer.html:

{
  "directed": true,
  "nodes": [{id, x, y, size, label, color, url?}, …],
  "edges": [{id, source, target, label, color, size, url?}, …]
}

The viewer parses node/edge attributes from the top level (NOT from a graphology-style attributes object); emitting the graphology serialization shape would make the viewer report "No nodes in sigma payload" even when the file has hundreds.

Colours are sourced exclusively from ASILPalette — the single source of truth shared with the SVG renderer. Sigma renders solid- coloured circles, so we pick the palette's edgeHex (the more saturated tone) for nodes; that reads best at the small node sizes the viewer uses.