[Fix] (MG_Backend/DirectVulkan): fence-backed GL sync objects and per-frame descriptor rewind

- Track every graphics-queue submission with a real fence: pooled fences
  for mid-frame flushes, the frame slot's fence for Present and readback.
  Completion advances a submit counter via vkGetFenceStatus polls,
  slot-fence waits, and device-idle points, and raises the buffer-manager
  serial floor from the frame serial each submission carried.
- GL sync objects now capture the submission index that will carry the
  commands recorded so far; ClientWaitSync honors
  GL_SYNC_FLUSH_COMMANDS_BIT with a mid-frame submit (gated on the index
  still being unsubmitted so poll loops cannot split the render pass), and
  blocking waits flush then vkWaitForFences with the caller timeout.
- FlushPendingCommands retires the submitted command buffer and restarts
  recording on a fresh one; retired buffers are freed once the slot fence
  is next waited, so an executing buffer is never reset.
- Rewind descriptor-set cursors exactly once per frame in Present (after
  the slot-fence wait), plus after the synchronous readback drain,
  replacing the ten lazy per-draw-path rewinds.

Verified: host tests 168/168, trace-replay 70/70.

MobileGL/MG_Backend/DirectVulkan/Renderer/VkBufferManager.cpp

@@ -129,6 +129,15 @@ namespace MobileGL::MG_Backend::DirectVulkan {
         }
     }
 
+    void VkBufferManager::NotifyFrameSerialComplete(Uint64 serial) {
+        // The current serial's work is still being recorded; a completion
+        // report for it (or beyond) can only come from a stale caller.
+        if (serial >= m_frameSerial) {
+            return;
+        }
+        m_completedSerialFloor = std::max(m_completedSerialFloor, serial);
+    }
+
     void VkBufferManager::SetCopyCommandProvider(IBufferCopyCommandProvider* provider) {
         m_copyProvider = provider;
     }