feat: add durable Kuknos settlement coordination

infrastructure/kuknos/submitter.go

@@ -0,0 +1,55 @@
+package kuknos
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/url"
+	"strings"
+
+	"gl/domain/ledger"
+)
+
+// Submitter sends a pre-signed Stellar/Kuknos transaction to Horizon. Signing
+// remains outside GL; GL owns durable admission, retry, and reconciliation.
+type Submitter struct {
+	Endpoint string
+	Client   *http.Client
+}
+
+func (s Submitter) Submit(ctx context.Context, record ledger.Settlement) (string, error) {
+	if s.Endpoint == "" || record.SignedTransactionXDR == "" {
+		return "", fmt.Errorf("kuknos submission endpoint and signed xdr are required")
+	}
+	form := url.Values{"tx": {record.SignedTransactionXDR}}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, strings.TrimRight(s.Endpoint, "/")+"/transactions", strings.NewReader(form.Encode()))
+	if err != nil {
+		return "", err
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	client := s.Client
+	if client == nil {
+		client = http.DefaultClient
+	}
+	resp, err := client.Do(req)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+	var body struct {
+		Hash   string `json:"hash"`
+		Extras struct {
+			ResultCodes struct {
+				Transaction string `json:"transaction"`
+			} `json:"result_codes"`
+		} `json:"extras"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&body); err != nil {
+		return "", fmt.Errorf("decode kuknos response: %w", err)
+	}
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 || body.Hash == "" {
+		return "", fmt.Errorf("kuknos rejected transaction: status=%d code=%s", resp.StatusCode, body.Extras.ResultCodes.Transaction)
+	}
+	return body.Hash, nil
+}