docs(proposal-112): add TopicPartitionRouter proposal

Add proposal for a Router implementation that presents multiple upstream
Kafka clusters to clients as a single virtual cluster, routing requests
based on topic ownership.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Tom Bentley <tbentley@redhat.com>

Author: tombentley