System recap (visual plan / visual recap)

Produce a high-altitude, visual review aid directly in the PR description. No deployment, no third-party service: GitHub renders the block (including mermaid diagrams), and the PR itself is the storage. A future viewer app can ingest the same marker-delimited block via the GitHub API, so follow the format exactly.

The recap is informational and non-blocking. It supplements the PR description and normal code review; it never replaces reading the diff.

Two modes, one format

• Plan mode (before/while implementing): describe the intended change against the current system. If no PR exists yet, put the block in the plan document or message; move it into the PR description once the PR exists.
• Recap mode (PR creation and every meaningful update): describe what the diff actually does. Replaces a plan-mode block if one exists.

Source-of-truth rules (non-negotiable)