Add the mesh's members to a list found in a file written into, and take back only those (hq ADR 0102)

internal/store/store.go

@@ -82,6 +82,10 @@ type Into struct {
 	Before  map[string]json.RawMessage `json:"before,omitempty"`
 	Absent  []string                   `json:"absent,omitempty"`
 	Created bool                       `json:"created,omitempty"`
+	// Added is, for each key whose declared value is a list, exactly the members the mesh added
+	// to the machine's list — never a member that was already there. Undeclared, only these go,
+	// and drift is judged on these alone (novox/hq ADR 0102).
+	Added map[string][]json.RawMessage `json:"added,omitempty"`
 }
 
 // State is the whole of what a node knows about what it has done.