happy path perf optimization

move goroutineID() inside the defer, guarded by !finished. It's safe — Goexit runs defers in the same goroutine, so the ID is stable.

pkg/result/result.go

@@ -151,7 +151,6 @@ func Async[T any](fn func() T) <-chan Expect[T] {
 			val      T
 			finished bool
 		)
-		id := goroutineID()
 		defer func() {
 			if v := recover(); v != nil {
 				if err, ok := v.(*stackError); ok {
@@ -165,7 +164,8 @@ func Async[T any](fn func() T) <-chan Expect[T] {
 				ch <- Ok(val)
 				return
 			}
-			if stored, ok := gErrors.LoadAndDelete(id); ok {
+			// goroutineID is looked up here, on the error path only.
+			if stored, ok := gErrors.LoadAndDelete(goroutineID()); ok {
 				ch <- Fail[T](stored.(error))
 			} else {
 				ch <- Fail[T](errors.New("goroutine exited unexpectedly"))