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/produce.go

@@ -0,0 +1,41 @@
+package kafka
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/twmb/franz-go/pkg/kgo"
+)
+
+// Producer writes to manually-partitioned shard topics: the caller has
+// already computed the destination partition via PartitionWithinShard, so
+// the client must be configured with a manual partitioner that respects
+// Record.Partition rather than re-hashing the key itself.
+type Producer struct {
+	client *kgo.Client
+}
+
+func NewProducer(brokers []string) (*Producer, error) {
+	cl, err := kgo.NewClient(
+		kgo.SeedBrokers(brokers...),
+		kgo.RecordPartitioner(kgo.ManualPartitioner()),
+	)
+	if err != nil {
+		return nil, fmt.Errorf("creating kafka producer client: %w", err)
+	}
+	return &Producer{client: cl}, nil
+}
+
+func (p *Producer) Close() {
+	p.client.Close()
+}
+
+// Produce synchronously writes one record to topic/partition and returns
+// the offset it landed at.
+func (p *Producer) Produce(ctx context.Context, topic string, partition int32, key, value []byte) (int64, error) {
+	rec := &kgo.Record{Topic: topic, Partition: partition, Key: key, Value: value}
+	if err := p.client.ProduceSync(ctx, rec).FirstErr(); err != nil {
+		return 0, fmt.Errorf("producing to %s/%d: %w", topic, partition, err)
+	}
+	return rec.Offset, nil
+}