docs: Document selecting the Rust vLLM gRPC servicer: smg serve --servicer-impl and SMG_VLLM_SERVICER_IMPL

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What this PR does

How do I run vLLM gRPC workers on the Rust servicer implementation — which smg serve flag or worker environment variable selects it, what is the default, which backend/connection-mode combinations are valid, and what must be installed for the selection to take effect instead of silently running Python?

Why we need it

Source change: smg-project/smg@1a32255

Commit 5 (#2727, 1a322557, 2026-10-02) added a launch-time switch between two implementations of the vLLM gRPC servicer, verified on current main. bindings/python/src/smg/serve.py: add_serve_args declares --servicer-impl with default python and choices python/rust (lines ~773-782: 'rust keeps the same vLLM gRPC contract but serves it from Rust over a same-host ZMQ engine connection, selected through SMG_VLLM_SERVICER_IMPL in the worker's environment; requires --backend vllm --connection-mode grpc'); VllmWorkerLauncher.gpu_env (lines 268-281) sets SMG_VLLM_SERVICER_IMPL=rust in each worker's environment only for connection_mode grpc + servicer_impl rust, and otherwise pops an inherited value ('The flag is authoritative'); parse_serve_args (lines ~880-891) rejects --servicer-impl rust for any backend/connection-mode other than vllm/grpc and calls _require_rust_servicer_hook (lines ~835-844), which fails fast when smg-grpc-servicer is not importable or the installed vLLM lacks the hook. grpc_servicer/smg_grpc_servicer/vllm/rust.py: SERVICER_IMPL_ENV="SMG_VLLM_SERVICER_IMPL" (line 55); resolve_servicer_impl (lines 451-469) reads args.servicer_impl, else the env var, else python; upstream_hook_installed (lines 486-511) scans the installed vLLM's entrypoints tree for the resolve_servicer_impl hook; require_upstream_hook (lines 517-525) refuses a rust selection on a vLLM without the hook; require_python_impl (lines 472-481) makes the Python servicer itself refuse to start when the flag asks for rust, so misconfiguration fails loudly rather than silently serving Python. The env-var path works on upstream's own entrypoint unchanged: SMG_VLLM_SERVICER_IMPL=rust python -m vllm.entrypoints.grpc_server ... (grpc_servicer/README.md section 'Rust request path (SMG_VLLM_SERVICER_IMPL=rust)'), and Rust mode additionally needs the smg wheel, which carries the smg.servicer.VllmGrpcServer binding (bindings/python/src/smg/servicer.py). The Router cannot tell the two implementations apart (rust.py module doc, line 13), and the Python implementation stays the default. Tests pin the CLI contract: test_rust_servicer_is_a_worker_env_flag_not_a_command and test_python_servicer_clears_an_inherited_rust_flag (bindings/python/tests/test_serve.py). Docs gap: a repo-wide grep for servicer-impl, SMG_VLLM_SERVICER_IMPL, VllmGrpcServer, and engine_servicer finds nothing in src/lib/content; getting-started/index.md's smg serve options table (lines 166-176) lists --backend and --connection-mode but not --servicer-impl, and getting-started/grpc-workers.md's 'Start a gRPC Worker' vLLM tab (lines 44-51) shows only the default Python-servicer start. Version caveat: the commit landed after the v1.11.0 bump (commit 37, #2670, 2026-09-24), so the edit must present the flag as current-main behavior without promising a release. This is one independent concern: one commit's launch-time selection surface (flag, env var, default, valid combinations, fail-fast requirements), documented where vLLM gRPC workers are started. The Rust servicer's runtime serving contract and the --mm-processor smg media mode it enables (commit 3, #2741) are the grpc-multimodal scan's charter — and the only page enumerating --mm-processor values (concepts/architecture/multimodal.md:117) is locked by open PR #103 — so they are deliberately excluded. No open or closed PR mentions the Rust servicer.

Scope: cli-configuration / vllm-rust-servicer-selection. Other concerns are deferred.

How to test

• Passed the documentation path and size guard (under 1000 added plus deleted lines; no file-count limit).
• Passed an independent accuracy and single-concern review.
• Passed git diff --check and pnpm check and the production pnpm build.

Checklist

• Tests added/updated (if applicable)
• Docs updated (if applicable)
• Commit signed off by XinyueZhang369 zoeyzhang369@gmail.com

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