feat(render): dynamic OpenGL/Vulkan backend split and backend-neutral viewer

- Extract libolive-rendercore static library to minimize backend link boundary.
- Add DynamicRenderer adapter with C ABI (oakgl/oakvulkan shared libs).
- Make OAK_ENABLE_DYNAMIC_RENDER_BACKEND default ON with OpenGL fallback.
- Implement VulkanRenderer prototype (textures, shaders, UBO blit, readback).
- Add backend-neutral viewer readback path (offscreen -> QImage -> QPainter).
- Refactor PluginRenderer to be renderer-agnostic; OFX plugins fall back to CPU
  path on non-OpenGL backends while preserving OpenGL render path.
- Add Renderer::AttachOutputTexture/DetachOutputTexture and C ABI forwards.
- Update docs/zh/render-backend-dynamic-plan.md for Phase 3/4/5.

app/render/backend/dynamicrenderer.h

@@ -44,6 +44,12 @@ public:
 								  const QPointF &pt) override;
 	virtual QOpenGLContext *OpenGLContext() const override;
 
+	virtual bool IsOpenGL() const override;
+
+	virtual void AttachOutputTexture(Texture *texture) override;
+
+	virtual void DetachOutputTexture() override;
+
 protected:
 	virtual void Blit(QVariant shader, AcceleratedJob &job,
 				  Texture *destination, VideoParams destination_params,
@@ -84,6 +90,8 @@ private:
 	OakBackendFlushFn flush_ = nullptr;
 	OakBackendGetPixelFromTextureFn get_pixel_from_texture_ = nullptr;
 	OakBackendBlitFn blit_ = nullptr;
+	OakBackendAttachOutputTextureFn attach_output_texture_ = nullptr;
+	OakBackendDetachOutputTextureFn detach_output_texture_ = nullptr;
 	OakBackendOpenGLContextFn opengl_context_ = nullptr;
 };