[Changho Hwang (chhwang)]

Summary

• Fix CudaIpcConnection::updateAndSync applying its byte offset twice; the copy and log now use the already offset destination pointer.
• Fix EthernetConnection::write counting the initial frame header as payload progress, which skipped bytes on chunked writes.
• Integrate the regression checks into existing CommunicatorTest.BasicWrite and PortChannelOneToOneTest.PingPongEthernet. No standalone tests were added, and performance tests and production staging-buffer defaults are unchanged.

Regression coverage

• BasicWrite exercises nonzero byte offsets only on existing CudaIpc connections; IB connections skip that per-edge phase while all ranks keep the same barriers. Per-sender slots and a guard at the former 9 * dstOffset target are checked. A uniform capacity guard skips only this added phase if its largest guard address will not fit the fixture buffer.
• PingPongEthernet retains its original ping-pong exercise, then uses the existing bootstrap and memory-pair helpers with a 64-byte staging buffer to verify a 128-byte chunked write followed by a 16-byte frame. It follows the existing two-participant fixture; additional world ranks take the fixture's existing nonparticipant path.

Validation

• Bounded CPU source-extract checks with fake GPU/socket boundaries passed under ASan/UBSan for CudaIpc offset 8 and Ethernet payloads of 48, 49, 128, and 16 bytes across four frames. These are not full-library or real-transport tests.
• clang-format --dry-run --Werror and git diff --check passed.
• The CUDA/MPI integration tests were not compiled or run: CMake, nvcc, mpicxx, and mpirun are unavailable. No GPU, MPI, socket, or real transport execution was performed.
• No benchmark was run. The fixes add no per-chunk operations, locks, allocations, or synchronization; large transfers may require extra sends to carry bytes the old path omitted. No measured no-regression claim is made.

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