Log HTTP/2 GOAWAY in debug mode

[kevmo314]

Summary

• add opt-in client-side logging for HTTP/2 GOAWAY frames when LUPINE_DEBUG or existing LUPINE_TRACE is enabled
• include the GOAWAY error code and debug payload, so demo timeout closes can explain why the remote device disappeared
• keep existing CUDA/NVML return behavior unchanged (CUDA_ERROR_DEVICE_UNAVAILABLE / existing fallback paths)

Verification

• cmake -S . -B build-goaway-debug -DCMAKE_LIBRARY_PATH=/usr/local/cuda/lib64/stubs
• cmake --build build-goaway-debug --parallel 2 --target lupine_driver lupine_nvml

Related

• Pairs with lupinemachines/demo-server#1, which sends GOAWAY(CANCEL, "lupine:demo-timeout") before closing timed-out demo sessions.