test(progress): read the log buffer safely while the heartbeat writes to it

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/progress/tracker_test.go

@@ -1,7 +1,6 @@
 package progress_test
 
 import (
-	"bytes"
 	"context"
 	"errors"
 	"log/slog"
@@ -60,20 +59,38 @@ func (f *fakeSink) beatCount() int {
 	return f.beats
 }
 
+// syncBuf is a log sink safe to read while the heartbeat goroutine is still writing.
+type syncBuf struct {
+	mu sync.Mutex
+	b  strings.Builder
+}
+
+func (s *syncBuf) Write(p []byte) (int, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.b.Write(p)
+}
+
+func (s *syncBuf) String() string {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.b.String()
+}
+
 // clock is a hand-wound clock so durations and throttling are exact.
 type clock struct{ t time.Time }
 
 func (c *clock) now() time.Time       { return c.t }
 func (c *clock) tick(d time.Duration) { c.t = c.t.Add(d) }
-func newClock() *clock { return &clock{t: time.Date(2026, 10, 2, 8, 0, 0, 0, time.UTC)} }
+func newClock() *clock                { return &clock{t: time.Date(2026, 10, 2, 8, 0, 0, 0, time.UTC)} }
 
 func plan() []progress.Planned {
 	return []progress.Planned{{Key: "audio", Label: "Preparing audio"}, {Key: "decide", Label: "Deciding"}, {Key: "save", Label: "Saving"}}
 }
 
-func newTracker(t *testing.T) (*progress.Tracker, *fakeSink, *clock, *bytes.Buffer) {
+func newTracker(t *testing.T) (*progress.Tracker, *fakeSink, *clock, *syncBuf) {
 	t.Helper()
-	sink, clk, buf := &fakeSink{}, newClock(), &bytes.Buffer{}
+	sink, clk, buf := &fakeSink{}, newClock(), &syncBuf{}
 	log := slog.New(slog.NewTextHandler(buf, &slog.HandlerOptions{Level: slog.LevelDebug}))
 	return progress.NewTracker(context.Background(), sink, log, plan(), progress.WithClock(clk.now), progress.WithMinSave(2*time.Second)), sink, clk, buf
 }