Renders our ass-renderer output alongside ffmpeg's libass reference into a single stacked comparison video. Uses tempfile for the intermediate render, a single ffmpeg invocation with subtitles+drawtext filter_complex, and supports hstack/vstack layout, time ranges, and CRF quality control. also fix regressions in drawing and animated text layout as well as a *very* confusing one in font resolution, where as far as we could tell the logic hadn't changed, but the behaviour demonstrably had.