Reduce city stutter: lower spawn rate, resync interval, M2 render distance

- MAX_SPAWNS_PER_FRAME 8→3 (each spawn does sync M2 load, 5-50ms each)
- Creature resync scan interval 1s→3s (O(N) entity iteration)
- M2 render distance: add 1000+ instance tier at 500 units, reduce 2000+
  tier from 350→300 units to cap draw call count in dense cities

src/core/application.cpp

@@ -786,7 +786,7 @@ void Application::update(float deltaTime) {
             if (gameHandler) {
                 static float creatureResyncTimer = 0.0f;
                 creatureResyncTimer += deltaTime;
-                if (creatureResyncTimer >= 1.0f) {
+                if (creatureResyncTimer >= 3.0f) {
                     creatureResyncTimer = 0.0f;
 
                     glm::vec3 playerPos(0.0f);