Fix waitGroup.Wait() blocking forever when Add is never called

When Wait() is called without any prior Add(), the counter is already 0
but the internal future was never resolved, causing Wait() to block
indefinitely. Return early from Wait() when the counter is 0.

Fixes #472

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people

internal/sync/waitgroup.go

@@ -21,6 +21,10 @@ func NewWaitGroup() WaitGroup {
 func (wg *waitGroup) Wait(ctx Context) {
 	wg.waiting = true
 
+	if wg.n == 0 {
+		return
+	}
+
 	if _, err := wg.f.Get(ctx); err != nil {
 		panic(err)
 	}

internal/sync/waitgroup_test.go

@@ -6,6 +6,22 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func Test_WaitGroup_WaitWithoutAdd(t *testing.T) {
+	s := NewScheduler()
+	ctx := Background()
+
+	wg := NewWaitGroup()
+
+	s.NewCoroutine(ctx, func(ctx Context) error {
+		wg.Wait(ctx)
+
+		return nil
+	})
+
+	s.Execute()
+	require.Equal(t, 0, s.RunningCoroutines())
+}
+
 func Test_WaitGroup_PanicsForInvalidCounters(t *testing.T) {
 	wg := NewWaitGroup()