perf(render): specialize singleton display spawns (#65)

Validated by paired GitHub JMH A/B run 29478305028 (PASS_OPTIMIZED, 8/8 wins for time and allocation).

Author: TypeThe0ry

src/main/java/top/ellan/mahjong/render/display/DisplayEntities.java

@@ -126,6 +126,10 @@ public static List<Entity> spawnAll(DisplayEntityRuntime runtime, List<EntitySpe
             return List.of();
         }
         DisplayEntityRuntime scopedRuntime = visibilitySnapshotRuntime(runtime);
+        if (specs.size() == 1) {
+            Entity entity = specs.get(0).spawn(scopedRuntime);
+            return entity == null ? List.of() : List.of(entity);
+        }
         List<Entity> spawned = new java.util.ArrayList<>(specs.size());
         for (EntitySpec spec : specs) {
             Entity entity = spec.spawn(scopedRuntime);