[jwildenhain]

Summary

• use Cairo PDF for ggplot2 output so styled fonts render in headless deployments
• expand worker regression coverage to the 18-cell plot type, engine, and format matrix
• add public HTTP MCP coverage for every plot-type/engine pair, all formats, orientations, scales, styles, custom colours, points, and means

Validation

• production stdio rendering matrix: 18/18 passed
• production public HTTP MCP matrix: 7/7 passed
• unsupported EPS output remains rejected