MCP tool: diff two rendered HTML states and report what changed and whether the change introduced accessibility regressions.
Lets an agent that just edited a LiveView ask "did I change rendered
behavior I didn't intend?" It returns the changed regions plus any
newly-introduced accessibility findings — content that updated without an
aria-live announcement, a control made keyboard-inaccessible, and so on
— and a risk tier (auto/review/block). Backed by
Excessibility.Review.