[MG_Backend] (P7): a respecified wire store starts with no submission stamp

- M2 round 2 left this to the integrator: lastUseSubmitIndex is B3's field and exists only on
  the integration tree; without the reset the fresh store answers busy-by-submission until the
  old index retires (a needless staged copy, conservative not unsafe)

MobileGL/MG_Backend/DirectVulkan/Renderer/VkBufferManager.cpp

@@ -248,6 +248,7 @@ namespace MobileGL::MG_Backend::DirectVulkan {
         DeferWireRelease(std::move(resource->buffer), orphanedUseSerial);
         resource->size = desc.Width;
         resource->lastUseSerial = 0;
+        resource->lastUseSubmitIndex = 0;  // M2 r2: the new store carries no submission yet (B3's stamp is per store)
         resource->gpuWritesPending = false;
         resource->stagedCoverage.clear();
         ++m_sliceEpochCounter;