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/fifo.go

@@ -0,0 +1,25 @@
+package queue
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	goredis "github.com/redis/go-redis/v9"
+
+	kmsvcredis "github.com/rockliang/kafka-management-service/internal/redis"
+)
+
+// acquireFIFOSlot claims the per-group exclusivity gate (design.md §3) so
+// ReceiveMessage never hands out two in-flight messages for the same
+// MessageGroupId at once. The lock's TTL matches the message's own
+// visibility timeout, so a lock left by a crashed/timed-out consumer
+// self-heals on the same schedule the reaper uses (design.md §5) — it is
+// also released early, by ack.lua/reap.lua, on ack/redeliver/DLQ-route.
+func acquireFIFOSlot(ctx context.Context, rdb *goredis.Client, queueName, groupID, receiptHandle string, visibilityTimeout time.Duration) (bool, error) {
+	ok, err := kmsvcredis.AcquireFIFOLock(ctx, rdb, queueName, groupID, receiptHandle, visibilityTimeout)
+	if err != nil {
+		return false, fmt.Errorf("acquire fifo slot %s/%s: %w", queueName, groupID, err)
+	}
+	return ok, nil
+}