feat: complete source consumer API request generation (Gap 4.3)

Add BuildConsumerCreateRequest, BuildConsumerCreateSubject, and
GetDeliverySequence to SourceCoordinator, modelling Go's
setupSourceConsumer/trySetupSourceConsumer. Covers DeliverPolicy
resume-from-sequence, AckPolicy.None, push/flow-control/heartbeat
consumer fields, and the $JS.API.CONSUMER.CREATE.{name} subject format.

src/NATS.Server/JetStream/MirrorSource/SourceCoordinator.cs

@@ -84,6 +84,12 @@ public sealed class SourceCoordinator : IAsyncDisposable
     /// <summary>The source configuration driving this coordinator.</summary>
     public StreamSourceConfig Config => _sourceConfig;
 
+    /// <summary>
+    /// Current delivery sequence counter — number of messages successfully written to the target store.
+    /// Go reference: server/stream.go si.dseq field
+    /// </summary>
+    public ulong GetDeliverySequence => _deliverySeq;
+
     public SourceCoordinator(IStreamStore targetStore, StreamSourceConfig sourceConfig)
     {
         _targetStore = targetStore;
@@ -95,6 +101,47 @@ public sealed class SourceCoordinator : IAsyncDisposable
         });
     }
 
+    /// <summary>
+    /// Builds the consumer configuration that would be sent to the source stream to set up
+    /// a push consumer for consumption. This models the consumer create request generated by
+    /// Go's setupSourceConsumer / trySetupSourceConsumer.
+    /// Go reference: server/stream.go:3474-3720 (setupSourceConsumer, trySetupSourceConsumer)
+    /// </summary>
+    public ConsumerConfig BuildConsumerCreateRequest()
+    {
+        // Go: server/stream.go:3597-3598 — if ssi.FilterSubject != _EMPTY_ { req.Config.FilterSubject = ssi.FilterSubject }
+        var cfg = new ConsumerConfig
+        {
+            AckPolicy = AckPolicy.None,
+            Push = true,
+            FlowControl = true,
+            HeartbeatMs = (int)HeartbeatInterval.TotalMilliseconds,
+        };
+
+        if (!string.IsNullOrWhiteSpace(_sourceConfig.FilterSubject))
+            cfg.FilterSubject = _sourceConfig.FilterSubject;
+
+        // Go: server/stream.go:3573-3582 — resume from LastOriginSequence+1, or DeliverAll when starting fresh
+        if (LastOriginSequence == 0)
+        {
+            cfg.DeliverPolicy = DeliverPolicy.All;
+        }
+        else
+        {
+            cfg.DeliverPolicy = DeliverPolicy.ByStartSequence;
+            cfg.OptStartSeq = LastOriginSequence + 1;
+        }
+
+        return cfg;
+    }
+
+    /// <summary>
+    /// Returns the JetStream API subject used to create a consumer on the source stream.
+    /// Go reference: server/stream.go:3531 — $JS.API.CONSUMER.CREATE.{sourceName}
+    /// </summary>
+    public string BuildConsumerCreateSubject() =>
+        $"$JS.API.CONSUMER.CREATE.{_sourceConfig.Name}";
+
     /// <summary>
     /// Processes a single inbound message from the origin stream.
     /// This is the direct-call path used when the origin and target are in the same process.