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/renderbackend_c.h

@@ -27,7 +27,9 @@ enum OakRenderBackendCapability {
 	OAK_RENDER_BACKEND_CAP_SHADERS = 1ULL << 1,
 	OAK_RENDER_BACKEND_CAP_BLIT = 1ULL << 2,
 	OAK_RENDER_BACKEND_CAP_READBACK = 1ULL << 3,
-	OAK_RENDER_BACKEND_CAP_VIEWER_CONTEXT = 1ULL << 4
+	OAK_RENDER_BACKEND_CAP_VIEWER_CONTEXT = 1ULL << 4,
+	OAK_RENDER_BACKEND_CAP_INSTANCE = 1ULL << 5,
+	OAK_RENDER_BACKEND_CAP_DEVICE = 1ULL << 6
 };
 
 struct OakRenderBackendInfo {
@@ -81,6 +83,9 @@ typedef void (*OakBackendBlitFn)(OakRenderBackendHandle handle,
 								 void *destination,
 								 const void *destination_params,
 								 bool clear_destination);
+typedef void (*OakBackendAttachOutputTextureFn)(OakRenderBackendHandle handle,
+											const void *texture_id);
+typedef void (*OakBackendDetachOutputTextureFn)(OakRenderBackendHandle handle);
 typedef void *(*OakBackendOpenGLContextFn)(OakRenderBackendHandle handle);
 
 #ifdef __cplusplus