EXPLAIN how something works with a hosted, tappable flow diagram — a walkthrough,
a failure trace, an answer to "how does X work?", a hypothetical. Saves via
lazyweb_explain_flow to insert-only storage, hosted at /explainer/<id>/.
Trigger on: "explain this with a diagram", "walk me through how X works",
"diagram why this failed", "show me how the pieces fit", "trace what happened".
