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

src/lib/content/getting-started/grpc-workers.md

@@ -50,6 +50,8 @@ The vLLM, SGLang, TokenSpeed and MLX gRPC servers come from the `smg-grpc-servic
       --port 50051
     ```
 
+    This starts the default Python servicer. Setting `SMG_VLLM_SERVICER_IMPL=rust` in the worker's environment serves the same contract from Rust instead; see [Rust vLLM Servicer](#rust-vllm-servicer).
+
 === "SGLang"
 
     ```bash
@@ -100,6 +102,38 @@ The vLLM, SGLang, TokenSpeed and MLX gRPC servers come from the `smg-grpc-servic
 
     MLX workers are gRPC-only and run on Apple Silicon. They don't support constrained decoding: chat requests with a forced `tool_choice` or a `response_format` get a 400, as do requests with `n` greater than 1 or string `stop` sequences.
 
+### Rust vLLM Servicer
+
+vLLM gRPC workers can serve the same gRPC contract from a Rust implementation instead of the default Python one. The selection is newer than v1.11.0: it exists on current main but not in the v1.11.0 wheels or engine images. The switch is the `SMG_VLLM_SERVICER_IMPL` environment variable, read by upstream vLLM's gRPC entrypoint before it builds an engine, so the worker command stays the same:
+
+```bash
+SMG_VLLM_SERVICER_IMPL=rust python -m vllm.entrypoints.grpc_server \
+  --model meta-llama/Llama-3.1-8B-Instruct \
+  --host 0.0.0.0 \
+  --port 50051
+```
+
+In Rust mode, the vLLM engine runs headless in a spawned child process and the servicer talks to it over a same-host ZMQ connection; Python keeps only the process lifecycle. That ZMQ hop is inside the worker — the gateway still connects over `grpc://`, so this is not the [ZMQ direct backend](zmq-workers.md). The gateway cannot tell the two implementations apart.
+
+With `smg serve`, select it with `--servicer-impl rust`. The flag defaults to `python`, is valid only with `--backend vllm --connection-mode grpc` (any other backend or connection mode is rejected at startup), and is authoritative: it sets `SMG_VLLM_SERVICER_IMPL=rust` in each worker's environment, and with `--servicer-impl python` it removes an `SMG_VLLM_SERVICER_IMPL` inherited from your shell.
+
+```bash
+smg serve \
+  --backend vllm \
+  --connection-mode grpc \
+  --servicer-impl rust \
+  --model meta-llama/Llama-3.1-8B-Instruct \
+  --data-parallel-size 2
+```
+
+For the selection to take effect, the worker's environment needs:
+
+- the `smg` wheel, which carries the Rust servicer binding (`smg.servicer.VllmGrpcServer`)
+- `smg-grpc-servicer[vllm]`, which carries the switch and the Rust request path
+- a vLLM whose gRPC entrypoint consults the switch (`smg_grpc_servicer.vllm.resolve_servicer_impl`)
+
+Misconfiguration fails loudly instead of silently serving Python: `smg serve --servicer-impl rust` refuses to start when `smg-grpc-servicer` is not importable or the installed vLLM lacks the hook, and when `SMG_VLLM_SERVICER_IMPL=rust` reaches a worker whose vLLM never consulted it, the Python servicer itself refuses to start.
+
 ---
 
 ## Connect SMG

src/lib/content/getting-started/index.md

@@ -167,6 +167,7 @@ To connect vLLM to its engine core directly instead of through gRPC, add `--conn
 |--------|---------|-------------|
 | `--backend` | `sglang` | Inference backend: `sglang`, `vllm`, `trtllm`, or `tokenspeed`. The `SMG_DEFAULT_BACKEND` environment variable changes the default |
 | `--connection-mode` | `grpc` | Worker connection mode: `grpc`, `http`, or `zmq`. TensorRT-LLM supports only `grpc` and TokenSpeed only `zmq`; `zmq` also works with `vllm` |
+| `--servicer-impl` | `python` | Implementation of the vLLM gRPC servicer: `python` or `rust`. `rust` requires `--backend vllm --connection-mode grpc` and is newer than v1.11.0; see [Rust vLLM Servicer](grpc-workers.md#rust-vllm-servicer) |
 | `--host` | `127.0.0.1` | Router host |
 | `--port` | `8080` | Router port (`30000` with `--backend sglang`; see below) |
 | `--data-parallel-size`, `--dp-size` | `1` | Number of worker replicas, each on its own GPU slice |
@@ -294,6 +295,8 @@ Use these when workers are not started via `smg serve`. Each command starts one
       --port 8000
     ```
 
+    Setting `SMG_VLLM_SERVICER_IMPL=rust` in the gRPC worker's environment serves the same contract from the Rust servicer instead of the default Python one (newer than v1.11.0); see [Rust vLLM Servicer](grpc-workers.md#rust-vllm-servicer).
+
 === "SGLang"
 
     SGLang depends on `smg-grpc-servicer`, so gRPC mode works after `pip install sglang`.