fix read state in self-chat and stop leaking read-marker into it (#18)

Self-chat never showed Read: the outgoing message was promoted by
sendDisplayedMarkerIfNeeded, but a later XEP-0184 <received/> receipt
or a LMM correction overwrote .read with .delivered/.sent directly,
bypassing Delivery.merged. Mark self-addressed outgoing messages read
as soon as the server accepts them and merge every weaker status into
the ladder instead of overwriting.

The self-addressed read-marker carried a plaintext <body>, so clients
that do not know our namespace (Conversations, Monal) rendered it as a
"Сообщение прочитано" bubble in the self-chat. Service stanzas now ride
on their payload element only.

Refs #18

Scripts/verify.sh

@@ -421,6 +421,14 @@ grep -q 'syncReadState' Sources/Shared/Models/AppModel.swift || {
   echo "Read receipts must be synced to the user's other devices"
   exit 1
 }
+grep -q 'delivery = \$0.delivery.merged(with: .delivered)' Sources/Shared/Models/AppModel.swift || {
+  echo "Delivery receipts must never downgrade an already read message"
+  exit 1
+}
+if grep -q 'message.body = Self.readMarkerBody' Sources/Shared/XMPP/XMPPService.swift; then
+  echo "Read-marker service stanzas must not carry a plaintext body (it leaks into self-chat on other clients)"
+  exit 1
+fi
 grep -q 'markersPublisher' Sources/Shared/XMPP/XMPPService.swift || {
   echo "XEP-0333 displayed markers must mark messages as read"
   exit 1