Reference image is served from /api/decksample/reference/{n} (looks for decksample/{n}.png then public/decksample/{nn}.png). Structural contract: decksample/SLIDE_SPEC.json (v3 + visualNotes + requiredChartType).
/api/decksample/reference/
decksample/{n}.png
public/decksample/{nn}.png
decksample/SLIDE_SPEC.json