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.

cmd/mesh-controller/changeplan.go

@@ -105,9 +105,23 @@ func changePlanOf(repository, base, head string, r mergeReach, entries []invento
 		p.Machines = append(p.Machines, *machines[name])
 	}
 	p.Summary = summaryOf(p)
+	p.Class = changeClassOf(p, entries)
 	return p
 }
 
+// classNone is the class of a change that moves no module: nothing for a walk to move (novox/hq ADR 0313 §2).
+const classNone = "none"
+
+// changeClassOf is a change plan's class (novox/hq ADR 0313 §2): that of the slowest-budgeted module it moves,
+// itself or as a dependent — core when one is on the controller's own path or holds the mesh's resolver, leaf
+// otherwise, a module it adds a directory for included — and none when it moves nothing.
+func changeClassOf(p link.ChangePlan, entries []inventory.Entry) string {
+	if len(p.Moved) == 0 && len(p.Dependents) == 0 && len(p.New) == 0 {
+		return classNone
+	}
+	return classOfModules(append(append([]string{}, p.Moved...), p.Dependents...), entries)
+}
+
 // grantsStepWord names the grants step (novox/hq issue 490): the bus's user list sent alone to the machine
 // holding the bus ahead of a walk's gate. Not the bus step, which replaces the bus itself.
 const grantsStepWord = "grants step"