Update Project

Assets/Scripts/Core/GameStateManager.cs

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+using UnityEngine;
+namespace FlappyBird.Core
+{
+    /// <summary>
+    /// Pure C# service — central authority for all game-state transitions.
+    /// No MonoBehaviour, no Unity lifecycle. Injected via Zenject.
+    ///
+    /// Transition side-effects:
+    ///   Playing  -> Time.timeScale = 1  (unfreeze)
+    ///   GameOver -> Time.timeScale = 0  (freeze everything)
+    ///   Menu     -> Time.timeScale = 0  (frozen until game starts)
+    /// </summary>
+    public sealed class GameStateManager : IGameStateManager
+    {
+        // ── IGameStateManager ──────────────────────────────────────────────
+        /// <inheritdoc/>
+        public GameState CurrentState { get; private set; } = GameState.Menu;
+        /// <inheritdoc/>
+        public event System.Action<GameState> OnStateChanged;
+        // ── Constructor ────────────────────────────────────────────────────
+        public GameStateManager()
+        {
+            // Start frozen; gameplay begins only when ChangeState(Playing) is called.
+            Time.timeScale = 0f;
+        }
+        // ── IGameStateManager ──────────────────────────────────────────────
+        /// <inheritdoc/>
+        public void ChangeState(GameState newState)
+        {
+            if (CurrentState == newState) return;
+            CurrentState = newState;
+            ApplyTimeScale(newState);
+            OnStateChanged?.Invoke(newState);
+        }
+        // ── Private helpers ────────────────────────────────────────────────
+        /// <summary>
+        /// Only Playing runs at normal speed.
+        /// Menu and GameOver both freeze Time so no physics or Update logic runs.
+        /// </summary>
+        private static void ApplyTimeScale(GameState state)
+        {
+            Time.timeScale = state == GameState.Playing ? 1f : 0f;
+        }
+    }
+}