fix: tighten ScoutConnCollected encapsulation and add concurrency test

- Move error check before Snapshot() to avoid unnecessary allocation
- Update existing tests to use Snapshot() instead of direct field access
- Add TestConcurrentAddSnapshot to explicitly exercise the mutex

mtglib/internal/doppel/scout.go

@@ -61,10 +61,14 @@ func (s Scout) learn(ctx context.Context, url string) (ScoutResult, error) {
 		client.CloseIdleConnections()
 	}
 
+	if err != nil {
+		return ScoutResult{}, err
+	}
+
 	data, writeIndex := results.Snapshot()
 
-	if err != nil || len(data) == 0 {
-		return ScoutResult{}, err
+	if len(data) == 0 {
+		return ScoutResult{}, nil
 	}
 
 	var result ScoutResult

mtglib/internal/doppel/scout_conn_collected_test.go

@@ -1,6 +1,7 @@
 package doppel
 
 import (
+	"sync"
 	"testing"
 	"time"
 
@@ -16,8 +17,10 @@ func (suite *ScoutConnCollectedTestSuite) TestAddSingle() {
 	collected := NewScoutConnCollected()
 	collected.Add(tls.TypeApplicationData, 100)
 
-	suite.Len(collected.data, 1)
-	suite.Equal(byte(tls.TypeApplicationData), collected.data[0].recordType)
+	data, _ := collected.Snapshot()
+
+	suite.Len(data, 1)
+	suite.Equal(byte(tls.TypeApplicationData), data[0].recordType)
 }
 
 func (suite *ScoutConnCollectedTestSuite) TestAddTimestampsAreMonotonic() {
@@ -31,11 +34,52 @@ func (suite *ScoutConnCollectedTestSuite) TestAddTimestampsAreMonotonic() {
 	time.Sleep(time.Microsecond)
 	collected.Add(tls.TypeApplicationData, 100)
 
-	for i := 1; i < len(collected.data); i++ {
-		suite.True(collected.data[i].timestamp.After(collected.data[i-1].timestamp))
+	data, _ := collected.Snapshot()
+
+	for i := 1; i < len(data); i++ {
+		suite.True(data[i].timestamp.After(data[i-1].timestamp))
 	}
 }
 
+func (suite *ScoutConnCollectedTestSuite) TestConcurrentAddSnapshot() {
+	collected := NewScoutConnCollected()
+
+	var wg sync.WaitGroup
+
+	wg.Add(3)
+
+	go func() {
+		defer wg.Done()
+
+		for i := 0; i < 1000; i++ {
+			collected.Add(tls.TypeApplicationData, i)
+		}
+	}()
+
+	go func() {
+		defer wg.Done()
+
+		for i := 0; i < 100; i++ {
+			collected.MarkWrite()
+		}
+	}()
+
+	go func() {
+		defer wg.Done()
+
+		for i := 0; i < 1000; i++ {
+			data, _ := collected.Snapshot()
+			_ = len(data)
+		}
+	}()
+
+	wg.Wait()
+
+	data, writeIndex := collected.Snapshot()
+	suite.Len(data, 1000)
+	suite.GreaterOrEqual(writeIndex, 0)
+}
+
 func TestScoutConnCollected(t *testing.T) {
 	t.Parallel()
 	suite.Run(t, &ScoutConnCollectedTestSuite{})