Diff two renders

1. Work out the two sides from the user's request (absolute paths):
   • two folders: project_a is the earlier or reference render, project_b the later one;
   • one folder, preview against final: pass it as both, with quality_a: "preview" and quality_b: "final". Both sides need a render; if one has none, say so and offer the render skill.
2. Call mcp__plugin_video-studio_engine__diff {project_a, project_b} (plus quality_a / quality_b when given).
3. Report, in this order:
   • Identical or not.
   • Spec: scenes added, removed and changed, then the notable field changes with before and after (the full list is in the report).
   • Lock: changes grouped by class. creative means the content changed; renderer means a tool, renderer, voice or font version changed (the likely cause when the spec did not change but frames did); spec means a platform contract changed; asset means a project input changed; metadata means only outputs such as post copy changed. If the lock was not compared (a side has no dist/video.lock, or it is for another quality), say so and suggest re-exporting that side.
   • Frames: how many sampled frames differ, with their SSIM. Read the side-by-side images it names under qa/diff-frames/ of project b (A | B | difference) and describe the visible change.
4. Connect the three: for example, frames differ but only a renderer version changed, so the change is the renderer, not the content.

To watch the change rather than read about it, the compare skill builds a before/after page of the same two renders (side by side, stacked or wipe, played in sync) for the user to open in a browser.

The full report is in project b's qa/diff.md (machine-readable: qa/diff.json). Diff never changes either project apart from those reports.