cmd/vet: add check for sync.WaitGroup abuse

// This program demonstrates the sync.WaitGroup misuse that the vet tool
// now flags. The pattern is:
//   go func() { wg.Add(1); defer wg.Done() }()
// The Add happens after the goroutine starts, which can race with wg.Wait
// and may cause the program to exit before the goroutine finishes.
package main

import (
	"fmt"
	"sync"
	"time"
)

func main() {
	var wg sync.WaitGroup
	// Defer Wait so the main goroutine waits for all workers.
	defer wg.Wait()

	// Start a goroutine that adds to the WaitGroup after it has begun.
	go func() {
		// Add is called after the goroutine has already started.
		wg.Add(1)
		defer wg.Done()
		// Simulate work.
		time.Sleep(100 * time.Millisecond)
		fmt.Println("Worker finished")
	}()

	// Sleep briefly to allow the goroutine to start before main exits.
	time.Sleep(10 * time.Millisecond)
	fmt.Println("Main exiting")
}