Suffix Cassandra design doc title with ": persist only"

The doc covers the shipped persist-only compare-and-set mode (the delete
fence is a deferred-design appendix), so the title now says so. Also
cross-link the Cassandra doc from the Kafka design doc intro, mirroring
the link the Cassandra doc already has in the other direction.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0195c6xdtRzSHPFobiPmmvfw

Author: tobiajo

docs/cassandra-single-writer-design.md

@@ -1,7 +1,7 @@
 ---
 id: cassandra-single-writer-design
-title: Cassandra single-writer design
-sidebar_label: Cassandra single-writer design
+title: "Cassandra single-writer design: persist only"
+sidebar_label: "Cassandra single-writer design: persist only"
 ---
 
 Design notes for the compare-and-set snapshot mode of `kafka-flow-persistence-cassandra`

docs/kafka-single-writer-design.md

@@ -5,7 +5,9 @@ sidebar_label: Kafka single-writer design
 ---
 
 Design notes for the transactional snapshot mode of `kafka-flow-persistence-kafka`
-(`KafkaPersistenceModuleOf.cachingTransactional`) — the mechanism and the measurements behind it.
+(`KafkaPersistenceModuleOf.cachingTransactional`) — the mechanism and the measurements behind it. The
+Cassandra backend solves the same problem differently — see
+[Cassandra single-writer design: persist only](cassandra-single-writer-design.md).
 
 ## Problem