docs: dependency-instability research and federation-relay plan

Research the failure-mode impact, project/release stability, and
operational-reliability data for the four core infra dependencies
(NATS/JetStream, MongoDB, Cassandra, Valkey), identifying the
request/reply-originated cross-site federation publish as the top
durability exposure, plus the implementation plan executed in the
following commit.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WcNmcyHTmyokFh9vYm3brj

Author: claude