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/renderer.h

@@ -29,12 +29,15 @@
 #include <memory>
 
 #include "common/define.h"
-#include "node/node.h"
-#include "render/colorprocessor.h"
-#include "render/job/colortransformjob.h"
+#include "render/shadercode.h"
 #include "render/videoparams.h"
 #include "texture.h"
-#include "job/pluginjob.h"
+
+// Forward declarations to keep the render core header lightweight
+namespace olive {
+class ColorTransformJob;
+class Node;
+}
 
 namespace olive
 {
@@ -117,6 +120,30 @@ public:
 		return lifetime_;
 	}
 
+	virtual bool IsOpenGL() const
+	{
+		return false;
+	}
+
+	/**
+	 * @brief Attach a texture as the current output destination for OFX plugin
+	 *        OpenGL rendering.
+	 *
+	 * Default implementation is a no-op. OpenGL-based renderers override this
+	 * to bind the texture as a framebuffer render target.
+	 */
+	virtual void AttachOutputTexture(olive::Texture *texture)
+	{
+		(void)texture;
+	}
+
+	/**
+	 * @brief Detach the current OFX plugin OpenGL output texture.
+	 *
+	 * Default implementation is a no-op.
+	 */
+	virtual void DetachOutputTexture() {}
+
 protected:
 	virtual void Blit(QVariant shader, olive::AcceleratedJob& job,
 					  olive::Texture *destination,