Say the class of what a change plan moves, so a fix's budget follows its modules (issue 403)

The issue tracker holds a fix to the budget of the slowest-budgeted module it
moves, and gives none to a fix that moves no module (hq ADR 0313 §2). The
walk's class covers every merge a walk carries; the plan's class is the
change's own: core, leaf, or none.

internal/link/events.go

@@ -311,6 +311,9 @@ type ChangePlan struct {
 	Steps []string `json:"steps,omitempty"`
 	// Summary is the plan in one line, for a commit status.
 	Summary string `json:"summary"`
+	// Class is the class of the slowest-budgeted module the change moves (novox/hq ADR 0282 §1, ADR 0313 §2):
+	// core, leaf, or none when it moves no module at all — a fix that moves nothing has no fix budget.
+	Class string `json:"class,omitempty"`
 }
 
 // MachinePlan is one machine's part of a change plan.