Fix production documentation screenshot gate sampling

[orangebtc]

Summary

• sample rendered documentation screenshots at 64×40 instead of 32×20
• retain the existing color-diversity and source/render difference thresholds
• avoid false failures for valid low-contrast Medusa Admin screenshots

Verification

• npm run check
• production official documentation render gate (4/4 images)
• full real-sandbox screenshot release gate
• production audit: 0 vulnerabilities
• opaque-overlay tamper simulation remains rejected
• packed npm artifact SHA-256 remains f89250fe8f2bb90c9be381496d171da7f864598d32719518c6f3b99e044d775c