fix: lifecycle — clear shutdown flag on startup, document waiter goroutine

- ServiceStartup clears c.shutdown so Core supports restart cycles
- ServiceShutdown waiter goroutine documented as inherent to sync.WaitGroup

Co-Authored-By: Virgil <virgil@lethean.io>

pkg/core/runtime.go

@@ -33,6 +33,7 @@ func (r *ServiceRuntime[T]) Config() *Config { return r.core.Config() }
 
 // ServiceStartup runs OnStart for all registered services that have one.
 func (c *Core) ServiceStartup(ctx context.Context, options any) Result {
+	c.shutdown.Store(false)
 	startables := c.Startables()
 	if startables.OK {
 		for _, s := range startables.Value.([]*Service) {
@@ -54,7 +55,9 @@ func (c *Core) ServiceShutdown(ctx context.Context) Result {
 	c.shutdown.Store(true)
 	c.ACTION(ActionServiceShutdown{})
 
-	// Drain background tasks before stopping services
+	// Drain background tasks before stopping services.
+	// On timeout, the waiter goroutine persists until tasks complete —
+	// this is inherent to sync.WaitGroup (no cancel mechanism).
 	done := make(chan struct{})
 	go func() {
 		c.wg.Wait()