Reviewing ffmpeg-skill Changes

Review a change to the ffmpeg-skill repository for the failures its own contract makes possible — a claim in a result document that is true at one layer and false at the layer a caller reads, a new flag that reaches the code but not the contract/docs/demo surfaces, a "bug" that docs/design-decisions.md already decided with a pinning test, a fix with no regression test, a new runtime dependency or a raw ffmpeg shell call that breaks the scope boundary, a SKILL.md line added without one trimmed, a review that re-derives findings from a summary instead of the tree at the reviewed commit. Use when asked to review a PR or diff in this repository, when authoring code here and n…