Renders the failure modes in the model IR as a Mermaid diagram — making the gap firebreak hunts for visible: which external caller blocks on :noproc when a process in another supervision subtree restarts.

Unlike Firebreak.Viz (which draws the whole coupling graph), this is failure-centric: it shows only the supervisors that have an inbound cross-tree crossing, their children (the restart blast set), and the crossing edges from outside. A dashed red :noproc edge is the dangerous case — a synchronous caller that blocks when the target restarts; async crossings are drawn faintly (a dropped cast, not a block). Target nodes are coloured by severity, so the picture doubles as a blast-radius heatmap.

A pure function of the model IR — mix firebreak --format failure.