feat(cluster): add placement engine with topology-aware peer selection (B9-prep)

src/NATS.Server/JetStream/Cluster/PlacementEngine.cs

@@ -0,0 +1,80 @@
+namespace NATS.Server.JetStream.Cluster;
+
+/// <summary>
+/// Topology-aware peer selection for stream/consumer replica placement.
+/// Go reference: jetstream_cluster.go:7212 selectPeerGroup.
+/// </summary>
+public static class PlacementEngine
+{
+    /// <summary>
+    /// Selects peers for a new replica group based on available nodes, tags, and cluster affinity.
+    /// Filters unavailable peers, applies cluster/tag/exclude-tag policy, then picks the top N
+    /// peers ordered by available storage descending.
+    /// </summary>
+    public static RaftGroup SelectPeerGroup(
+        string groupName,
+        int replicas,
+        IReadOnlyList<PeerInfo> availablePeers,
+        PlacementPolicy? policy = null)
+    {
+        // 1. Filter out unavailable peers.
+        IEnumerable<PeerInfo> candidates = availablePeers.Where(p => p.Available);
+
+        // 2. If policy has Cluster, filter to matching cluster.
+        if (policy?.Cluster is { Length: > 0 } cluster)
+            candidates = candidates.Where(p => string.Equals(p.Cluster, cluster, StringComparison.OrdinalIgnoreCase));
+
+        // 3. If policy has Tags, filter to peers that have ALL required tags.
+        if (policy?.Tags is { Count: > 0 } requiredTags)
+            candidates = candidates.Where(p => requiredTags.All(tag => p.Tags.Contains(tag)));
+
+        // 4. If policy has ExcludeTags, filter out peers with any of those tags.
+        if (policy?.ExcludeTags is { Count: > 0 } excludeTags)
+            candidates = candidates.Where(p => !excludeTags.Any(tag => p.Tags.Contains(tag)));
+
+        // 5. If not enough peers after filtering, throw InvalidOperationException.
+        var filtered = candidates.ToList();
+        if (filtered.Count < replicas)
+            throw new InvalidOperationException(
+                $"Not enough peers available to satisfy replica count {replicas}. " +
+                $"Available after policy filtering: {filtered.Count}.");
+
+        // 6. Sort remaining by available storage descending.
+        var selected = filtered
+            .OrderByDescending(p => p.AvailableStorage)
+            .Take(replicas)
+            .Select(p => p.PeerId)
+            .ToList();
+
+        // 7. Return RaftGroup with selected peer IDs.
+        return new RaftGroup
+        {
+            Name = groupName,
+            Peers = selected,
+        };
+    }
+}
+
+/// <summary>
+/// Describes a peer node available for placement consideration.
+/// Go reference: jetstream_cluster.go peerInfo — peer.id, peer.offline, peer.storage.
+/// </summary>
+public sealed class PeerInfo
+{
+    public required string PeerId { get; init; }
+    public string Cluster { get; set; } = string.Empty;
+    public HashSet<string> Tags { get; init; } = new(StringComparer.OrdinalIgnoreCase);
+    public bool Available { get; set; } = true;
+    public long AvailableStorage { get; set; } = long.MaxValue;
+}
+
+/// <summary>
+/// Placement policy specifying cluster affinity and tag constraints.
+/// Go reference: jetstream_cluster.go Placement struct — cluster, tags.
+/// </summary>
+public sealed class PlacementPolicy
+{
+    public string? Cluster { get; set; }
+    public HashSet<string>? Tags { get; set; }
+    public HashSet<string>? ExcludeTags { get; set; }
+}