fix: resolve nil pubsub preventing live game updates

v.PubSub() was captured at startup before v.Start() initialized NATS,
so both stores held nil and notify() silently no-oped. Replace the
PubSub interface with a callback that evaluates v.PubSub() lazily at
call time.

snake/store.go

@@ -6,10 +6,6 @@ import (
 	"sync"
 )
 
-type PubSub interface {
-	Publish(subject string, data []byte) error
-}
-
 type Persister interface {
 	SaveSnakeGame(sg *SnakeGame) error
 	LoadSnakeGame(id string) (*SnakeGame, error)
@@ -21,8 +17,8 @@ type Persister interface {
 type SnakeStore struct {
 	games     map[string]*SnakeGameInstance
 	gamesMu   sync.RWMutex
-	persister Persister
-	pubsub    PubSub
+	persister  Persister
+	notifyFunc func(gameID string)
 }
 
 func NewSnakeStore() *SnakeStore {
@@ -35,14 +31,14 @@ func (ss *SnakeStore) SetPersister(p Persister) {
 	ss.persister = p
 }
 
-func (ss *SnakeStore) SetPubSub(ps PubSub) {
-	ss.pubsub = ps
+func (ss *SnakeStore) SetNotifyFunc(f func(gameID string)) {
+	ss.notifyFunc = f
 }
 
 func (ss *SnakeStore) makeNotify(gameID string) func() {
 	return func() {
-		if ss.pubsub != nil {
-			ss.pubsub.Publish("snake."+gameID, nil)
+		if ss.notifyFunc != nil {
+			ss.notifyFunc(gameID)
 		}
 	}
 }