feat(queue): add message-plane core logic for send/receive/delete/visibility

Implements SendMessage (size cap, FIFO dedup, shard routing), ReceiveMessage
(long-poll loop across active+closing shards, FIFO per-group exclusivity
gating, redelivery hand-out), DeleteMessage (ack + low-watermark offset
advancement), and ChangeMessageVisibility against the Redis state layer
and a real Kafka producer/consumer.

internal/core/queue/change_visibility.go

@@ -0,0 +1,29 @@
+package queue
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	goredis "github.com/redis/go-redis/v9"
+
+	kmsvcredis "github.com/rockliang/kafka-management-service/internal/redis"
+)
+
+type ChangeVisibilityService struct {
+	Redis *goredis.Client
+}
+
+// ChangeMessageVisibility implements ChangeMessageVisibility (design.md
+// §2b): extends (or shortens) how long a received message stays invisible
+// to other consumers, without affecting its receive count.
+func (s *ChangeVisibilityService) ChangeMessageVisibility(ctx context.Context, queueName, receiptHandle string, newTimeout time.Duration) error {
+	ok, err := kmsvcredis.ExtendVisibility(ctx, s.Redis, queueName, receiptHandle, newTimeout)
+	if err != nil {
+		return fmt.Errorf("change visibility %s: %w", queueName, err)
+	}
+	if !ok {
+		return fmt.Errorf("receipt handle not found or already expired")
+	}
+	return nil
+}