Derive data protection from a module's declared data (hq ADR 0233)

A module's data section says what it keeps and how precious it is; the backup holder's lines,
binding stickiness, retirement on unassign and D13's conditions follow from it, so issue 273's
empty replacement is said and an unassigned module's data is remembered, not forgotten.

cmd/mesh-controller/acts.go

@@ -224,6 +224,10 @@ func unassign(ctx context.Context, open *stores, node string, modules ...string)
 	for _, line := range unheldChange(shelf, node, assigned, left) {
 		answer += "\n  " + line
 	}
+	// What it leaves behind that is irreplaceable is kept and retired, never removed (novox/hq ADR 0233).
+	for _, line := range keptOnUnassign(ctx, open.inventory, node, modules) {
+		answer += "\n  " + line
+	}
 	return answer + blockedElsewhere(ctx, open, node), nil
 }