Add batch consumer partition assignment strategy - #294
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Problem
Batch consumers use Kafka's default RangeAssignor (eager rebalancing): every rebalance — deploy, restart, or scaling — makes all consumers revoke all partitions before reassigning ("stop-the-world"). There was no supported way to switch to cooperative rebalancing (CooperativeStickyAssignor).
Solution
New opt-in, per-route flag :partition-assignment-strategy. Omit it → unchanged default behavior (no impact on existing consumers). Generic by design — supports any assignor:
:partition-assignment-strategy :cooperative-sticky ; short name
:partition-assignment-strategy "org.apache.kafka.clients.consumer.CooperativeStickyAssignor" ; FQCN
:partition-assignment-strategy [:range :cooperative-sticky] ; ordered preference list (migration)
Short names: :range, :round-robin, :sticky, :cooperative-sticky. Unknown values fail loudly at startup.
Eager → cooperative is protocol-incompatible; do a two-phase rolling deploy: phase 1 [:range :cooperative-sticky], then (after full rollout) phase 2 :cooperative-sticky. New groups can use :cooperative-sticky directly. Runbook in doc/kafka_produce_consume.md.