Move barriers and frame submit to synchronization2

Image barriers now carry their own stage masks and are recorded with
pipelineBarrier2, so barriers flushed together no longer share the
union of their stages. AccessState uses the 64-bit stage and access
flags, and the frame submit uses submit2.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

engine/src/renderer/render_graph.hpp

@@ -104,9 +104,7 @@ class RenderGraph {
   };
 
   struct PendingBarriers {
-    std::vector<vk::ImageMemoryBarrier> barriers;
-    vk::PipelineStageFlags srcStages;
-    vk::PipelineStageFlags dstStages;
+    std::vector<vk::ImageMemoryBarrier2> barriers;
   };
 
   void transitionTo(ImageRef ref, const AccessState& next,