[yhl-amd]

What this PR does / why we need it:

Add an opt-in chunk-store server module that returns a completion event for each chunk, allowing explicit RPC clients to reuse source KV blocks as their transfers finish. The module loads through --server-module, reuses the existing transfer module, and exposes a typed RPC over ZMQ and gRPC. Includes unit tests, cross-process GPU tests, and usage/design documentation.

Special notes for your reviewers:

This PR is entirely within yhl-amd/LMCache. Its base, codex/server-module-plugins, is an exact copy of maobaolong's plugin branch at 352ea6bc56ff814c61df5be6332399465cbb0a29. The head adds one signed-off commit. Files changed contains only the chunk-store delta: 9 files, +723/-0 (190 lines of implementation/protocol, 453 test lines, 80 documentation lines). The feature patch is identical to the previously reviewed implementation.

Each chunk uses the ordinary store path independently: earlier chunks may remain committed if a later chunk fails. Event handles inherit the existing backend retention contract. Submission overhead has not been benchmarked.

Validation on head 74f63569e8fbbd94a9985fd33d6bb687d159a836, directly on the plugin baseline:

• 150 tests passed, including 4 real cross-process GPU cases (ZMQ/gRPC, token offsets 0/16) checking exact KV retrieval after per-chunk source overwrite.
• Ran on mi355-gpu-51 through Slurm job 7803, with one GPU; allocation released after completion. Python 3.10.12, PyTorch 2.10.0+rocm7.0, HIP 7.0.51831. Reused an existing native build; native extension sources are unchanged.
• Ruff, formatting, isort, mypy for the changed production files, codespell, SPDX, protobuf generation, and diff checks passed. The chunk-store documentation page builds with strict Sphinx checks.

====================== 150 passed, 150 warnings in 18.37s ======================

The warnings are the existing OpenTelemetry LoggingHandler deprecation.

Reproduce the test selection

python -m lmcache.v1.multiprocess.transport.grpc_impl._proto_gen._generate
python -m pytest -q \
  tests/v1/multiprocess/test_chunk_store.py \
  tests/v1/multiprocess/test_chunk_store_gpu.py \
  tests/v1/multiprocess/test_server_module.py \
  tests/v1/multiprocess/test_grpc_transport.py \
  tests/v1/multiprocess/test_event_ipc_handle_path.py \
  tests/v1/multiprocess/test_config.py \
  tests/v1/multiprocess/test_transport_factory.py \
  tests/v1/multiprocess/test_protocols.py

If applicable:

• this PR contains user facing changes - docs added
• this PR contains unit tests