Fix type assertion bug in handleClusterMessage

JSON unmarshal produces map[string]interface{}, not concrete types.
Added ModelPayload and MessagePayload concrete types that implement
RuntimeModel and RuntimeMessage interfaces respectively.

The handleClusterMessage now re-marshals and unmarshals the payload
to convert from map[string]interface{} to the proper concrete type.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cluster/types.go

@@ -147,3 +147,29 @@ type RuntimeMessage interface {
 	// GetType returns the message type identifier
 	GetType() string
 }
+
+// ModelPayload is a concrete type for JSON-unmarshaling RuntimeModel payloads.
+// Use this when receiving model data over the network.
+type ModelPayload struct {
+	ID   string `json:"id"`
+	Name string `json:"name"`
+}
+
+// GetID implements RuntimeModel
+func (m *ModelPayload) GetID() string { return m.ID }
+
+// GetName implements RuntimeModel
+func (m *ModelPayload) GetName() string { return m.Name }
+
+// MessagePayload is a concrete type for JSON-unmarshaling RuntimeMessage payloads.
+// Use this when receiving message data over the network.
+type MessagePayload struct {
+	TargetActorID string `json:"targetActorId"`
+	Type          string `json:"type"`
+}
+
+// GetTargetActorID implements RuntimeMessage
+func (m *MessagePayload) GetTargetActorID() string { return m.TargetActorID }
+
+// GetType implements RuntimeMessage
+func (m *MessagePayload) GetType() string { return m.Type }