feat(WebGPU): add MSAA (multi-sample anti-aliasing) support

Author: MaxKopacz

Sources/Rendering/WebGPU/RenderWindow/api.md

@@ -1,4 +1,29 @@
-WebGPU rendering window
+# WebGPU RenderWindow
 
-vtkWebGPURenderWindow is designed to view/render a vkRenderWindow
-using the WebGPU API
+vtkWebGPURenderWindow is designed to view/render a vtkRenderWindow
+using the WebGPU API.
+
+## Multi-Sample Anti-Aliasing (MSAA)
+
+MSAA can be enabled to improve image quality by reducing aliasing
+artifacts on geometry edges. When active, both opaque and translucent
+rendering passes use multisampled textures and resolve them before
+compositing.
+
+### sampleCount (set/get)
+
+Controls the number of MSAA samples per pixel.
+
+- **Default:** `1` (no anti-aliasing)
+- **Valid values:** `1` or `4`
+- Setting any other value is ignored and logs an error.
+- Changing `sampleCount` at runtime automatically recreates the
+  internal render encoders and textures on the next frame.
+
+**Usage:**
+
+```js
+const renderWindow = vtkWebGPURenderWindow.newInstance();
+renderWindow.setSampleCount(4); // enable 4× MSAA
+renderWindow.setSampleCount(1); // disable MSAA
+```