feat(redis): add shard-aware key schema, Lua scripts, and atomic ops

Implements the kmsvc: key schema (inflight, pending/watermark keyed by
shard+partition, vis_index, dedup, fifo_lock, queue meta, shard map) plus
the atomic reap/ack Lua scripts used for safe multi-replica redelivery
and DLQ routing.

internal/redis/dedup.go

@@ -0,0 +1,21 @@
+package redis
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/redis/go-redis/v9"
+)
+
+// TryDedup attempts to claim a MessageDeduplicationId for a FIFO MessageGroupId
+// (design.md §4 `kmsvc:dedup:` row). Returns true if this is the first send
+// within the dedup window (the message should be accepted), false if it's a
+// duplicate (the message should be silently deduped).
+func TryDedup(ctx context.Context, rdb *redis.Client, queue, groupID, dedupID string, window time.Duration) (bool, error) {
+	ok, err := rdb.SetNX(ctx, DedupKey(queue, groupID, dedupID), "1", window).Result()
+	if err != nil {
+		return false, fmt.Errorf("dedup check %s/%s: %w", groupID, dedupID, err)
+	}
+	return ok, nil
+}