Add cudaStream_t to GpuBuffers for async H2D/D2H transfers in BSSN and
Z4C substep functions. Adds cudaStreamSynchronize(0) before D2H to
enforce kernel/transfer ordering across streams, and a sync between
state and matter H2D uploads to prevent h_stage race on RK4==0.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>