What does image-story-assembly-hardening do?
Use when assembling a long (≥5 min / ≥40 shots) image-story or slideshow video with ffmpeg — push-in (Ken Burns / zoompan) motion, crossfades, burned-in or layered subtitles, and pre-delivery QC. Covers the four failure modes that only show up at this scale (multi-branch filtergraph OOM/throttle, crossfade subtitle ghosting, zoompan jitter, generation artifacts like scan-lines and baked watermarks) plus the quantitative criteria to catch them. 触发词：图片联播、静态图叙事、长片装配、推镜卡顿、转场字幕叠影、去水印、扫描线伪影、成片 QA。