Cassandra design doc: orient the reader to where the subtlety lives

The PR history's core learning (evolution-gaming#834): the
conditional write looked trivial, the pitfalls were all in not fencing
the legitimate owner. Say that up front so the doc's weight makes sense.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PGxViqVaLYb1xQyveED6Hs

Author: tobiajo

docs/cassandra-single-writer-design.md

@@ -10,6 +10,10 @@ enablement, costs and rollout guidance are in
 [Persistence](persistence.md#protecting-against-stale-snapshot-writes). The Kafka backend solves the
 same problem differently — see [Kafka single-writer design](kafka-single-writer-design.md).
 
+The conditional write itself is the easy part. Most of this document is about not fencing the
+*legitimate* owner: the delete path, the replay window and deleted-key recovery are where a
+lightweight-transaction guard that looks trivial turns out not to be.
+
 ## Problem
 
 [kafka-flow#732](https://github.com/evolution-gaming/kafka-flow/issues/732): during a rebalance a