Exempt only the user list's content from the grants step's guard, and test the guard in the send itself (review of #230)

The guard let any field of the list's resource move; its path or mode
changing is a change the step must not carry. The store test now calls
sendToEach, so removing the guard there fails it.

cmd/mesh-controller/grants_step_store_test.go

@@ -3,11 +3,13 @@ package main
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"strings"
 	"testing"
 	"time"
 
 	"github.com/novox/mesh-controller/internal/catalogue"
+	"github.com/novox/mesh-controller/internal/identity"
 	"github.com/novox/mesh-controller/internal/inventory"
 	"github.com/novox/mesh-controller/internal/link"
 )
@@ -169,4 +171,10 @@ func TestTheGrantsStepSendsTheUserListAndNothingElse(t *testing.T) {
 	if !strings.Contains(only, "wallpaper newly assigned") || !strings.Contains(only, "+wallpaper") {
 		t.Fatalf("a new assignment on the bus's machine reads %q; want the step refused, naming it", only)
 	}
+	// And the send itself refuses it, unsent: judged where it is composed, before the bus is dialled.
+	identity.ForTest(t)
+	if _, err := sendToEach(kept, open, []string{"anchor"}); !errors.Is(err, errNotGrantsOnly) ||
+		!strings.Contains(err.Error(), "wallpaper") {
+		t.Fatalf("the grants step's send of a new assignment was not refused by the send: %v", err)
+	}
 }