feat(eventbus): add platform runtimes and benchmarks

Add the Entities NativeQueue adapter, standalone .NET runtime and source generator, reproducible smoke coverage, and Unity benchmark assets for EventBus 2.0.

Assets/Modules/ShrinkEventBus.Entities/README.md

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+# ShrinkEventBus Entities
+
+`com.cneicy.shrink-eventbus-entities` 是可选 ECS/Burst 适配包。它不创建第二套事件总线，而是让 Burst Job 把 `unmanaged IShrinkEvent` 写入 `NativeQueue<T>`，随后由主线程或 ECS playback system 发布到指定的 `IShrinkEventBus`。
+
+```csharp
+using var queue = new ShrinkEcsEventQueue<MyEcsEvent>();
+var writer = queue.Writer;
+
+// Burst Job 中：writer.Post(in eventData);
+// Playback 阶段：
+queue.Playback(worldBus);
+```
+
+限制：Burst 端只负责事实事件生产，不能直接执行托管 handler、UniTask、取消或结果裁决。