feat(kafka): add shard-aware topology, admin, and client layer

Topic naming (kmsvc.{queue}.shard-{id} / .fifo.shard-{id} / .dlq.shard-{id}),
hash-range shard selection/splitting (murmur2-based, matching Kafka's
default partitioner), and topic admin/producer/consumer client
constructors shared by the operator and the message-plane.

internal/kafka/client.go

@@ -0,0 +1,37 @@
+package kafka
+
+import (
+	"fmt"
+
+	"github.com/twmb/franz-go/pkg/kgo"
+)
+
+// NewProducerClient builds a producer-only client. Callers own Close().
+func NewProducerClient(brokers []string) (*kgo.Client, error) {
+	cl, err := kgo.NewClient(
+		kgo.SeedBrokers(brokers...),
+		kgo.ProducerBatchCompression(kgo.SnappyCompression()),
+	)
+	if err != nil {
+		return nil, fmt.Errorf("creating kafka producer client: %w", err)
+	}
+	return cl, nil
+}
+
+// NewConsumerClient builds a client that consumes the given topics as part
+// of consumerGroup. The message-plane service relies on Kafka's native
+// consumer-group rebalancing for partition ownership across replicas
+// (design.md §9) — offset commit is driven manually via the low-watermark
+// strategy in design.md §3, so auto-commit is disabled.
+func NewConsumerClient(brokers []string, consumerGroup string, topics ...string) (*kgo.Client, error) {
+	cl, err := kgo.NewClient(
+		kgo.SeedBrokers(brokers...),
+		kgo.ConsumerGroup(consumerGroup),
+		kgo.ConsumeTopics(topics...),
+		kgo.DisableAutoCommit(),
+	)
+	if err != nil {
+		return nil, fmt.Errorf("creating kafka consumer client: %w", err)
+	}
+	return cl, nil
+}