Let a trusted file that names its keys hold only those keys at the terminal (hq issue 586, ADR 0329 §5)

A module with a trusted mergeable file had its whole layer refused through the
settings verb, because such a file took any key. Once a module's files name the
keys they take (ADR 0326), each file takes only those, so only a trusted file's
named keys are the terminal's: betterbird's tags, in a file nothing trusts, is set
through the verb, while its accounts and identities stay the terminal's or the
operator's warrant. A module whose files name no keys keeps the whole-layer rule.

cmd/mesh-controller/terminal_settings_test.go

@@ -345,6 +345,37 @@ func TestAnyKeyOfAModuleWithATrustedMergeableFileIsRefusedThroughAVerb(t *testin
 	}
 }
 
+// A module whose mergeable files name the keys they take (ADR 0326): a key of a trusted file is refused through a
+// verb, and a key of a file nothing trusts is set through it (novox/hq ADR 0329 §5, issue 586: betterbird's tags).
+func TestOnlyATrustedFilesNamedKeyIsRefusedThroughAVerb(t *testing.T) {
+	open := aMesh(t)
+	ctx := t.Context()
+	register(t, open, catalogue.Manifest{Module: "mailer", Version: "1",
+		Resources: []map[string]any{
+			{"id": "accounts", "type": "file", "path": "/var/lib/mailer/accounts.json", "mode": "0600", "merge": "json",
+				"trusted": true, "takes-settings": []any{"accounts"}, "content": `{"accounts": {}}`},
+			{"id": "tags", "type": "file", "path": "/var/lib/mailer/tags.json", "mode": "0644", "merge": "json",
+				"trusted": false, "takes-settings": []any{"tags"}, "content": `{"tags": {}}`}}})
+	if _, err := assign(ctx, open, "anchor", "mailer"); err != nil {
+		t.Fatal(err)
+	}
+	if err := throughVerb(t, "settings", map[string]any{"module": "mailer", "values": `{"tags":{"todo":{"name":"To do"}}}`}); err != nil {
+		t.Fatalf("a key of a file nothing trusts is the verb's: %v", err)
+	}
+	err := throughVerb(t, "settings", map[string]any{"module": "mailer", "replace": "true",
+		"values": `{"tags":{"todo":{"name":"To do"}},"accounts":{"mail":{"address":"me@mail.example"}}}`})
+	if err == nil || !strings.Contains(err.Error(), "controller's terminal") || !strings.Contains(err.Error(), "accounts") {
+		t.Fatalf("a key of a trusted file is the terminal's: %v", err)
+	}
+	values, _, err := open.inventory.Layer(ctx, "", "mailer")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if raw, _ := json.Marshal(values); string(raw) != `{"tags":{"todo":{"name":"To do"}}}` {
+		t.Fatalf("the layer holds the tags alone: %s", raw)
+	}
+}
+
 // Who owns a module's directory is said at the terminal alone (novox/hq issue 279): a directory's owner may name
 // the settings the module runs as, root chowns the folder to them at the next push, and through the settings verb
 // any caller could have set puid to 0 or to its own uid. The key is refused through every verb route even where a