Add batch_transfer_async_submit() (non-blocking submit -> batch_id) and
wait_batch_transfers() (block-until-all, graceful) to MooncakeTransferEngine,
exposing the mooncake async API for the upcoming per-layer overlapped transfer.
Deliberately avoids batch_transfer_*_on_cuda: its CUDA host callback busy-waits
on the stream and calls _exit(1) on transfer failure (verified in the mooncake
source transfer_engine_py.cpp), which is unsafe for production (a decode-side
abort mid-transfer would crash the prefill). The async submit + wait pair lets
per-layer submits pipeline in the RDMA engine, then waits once on a background
thread with graceful failure.
Both bindings raise a clear upgrade error if the engine predates the API and
degrade to -1 on transient errors. Mock-engine unit tested (8 cases).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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