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/opengl/openglbackend_c.cpp

@@ -19,6 +19,8 @@ public:
 	using olive::OpenGLRenderer::CreateNativeTexture;
 	using olive::OpenGLRenderer::DestroyInternal;
 	using olive::OpenGLRenderer::DestroyNativeTexture;
+	using olive::OpenGLRenderer::AttachTextureAsDestination;
+	using olive::OpenGLRenderer::DetachTextureAsDestination;
 };
 
 BackendOpenGLRenderer *Renderer(OakRenderBackendHandle handle)
@@ -176,3 +178,15 @@ OAK_RENDER_BACKEND_EXPORT void *oak_renderer_opengl_context(
 {
 	return Renderer(handle)->context();
 }
+
+OAK_RENDER_BACKEND_EXPORT void oak_renderer_attach_output_texture(
+	OakRenderBackendHandle handle, const void *texture_id)
+{
+	Renderer(handle)->AttachTextureAsDestination(VariantRef(texture_id));
+}
+
+OAK_RENDER_BACKEND_EXPORT void oak_renderer_detach_output_texture(
+	OakRenderBackendHandle handle)
+{
+	Renderer(handle)->DetachTextureAsDestination();
+}