Add GPU Lab configs, docs, and runbooks for VM303.

Mirror gpu-lab compose files and model-manager catalog from GPU-Dev; document Dockhand mapping and SGLang cutover target.

docs/runbook-vm303.md

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+# GPU Lab Runbook — VM303 (atc-gpu-dev)
+
+See also `config/gpu-lab/RUNBOOK.md` (synced from VM303).
+
+## Activate via UI
+
+GPU Lab → select model → **Activate**. Writes `/root/gpu-lab/active/current.env`, stops other gpu-lab containers, starts matching compose stack.
+
+## Manual activate (SGLang + Qwen 3.6 27B AWQ)
+
+```bash
+cd /root/gpu-lab
+cat > active/current.env << 'EOF'
+MODEL_PATH=/models/qwen36-27b-awq
+SERVED_NAME=qwen3.6-27b-awq
+TP_SIZE=4
+GPU_MEM_UTIL=0.9
+MAX_MODEL_LEN=8192
+EOF
+docker compose -f compose.vllm-legacy.yml down
+docker compose -f compose.vllm-modern.yml down
+docker compose -f compose.sglang.yml up -d
+curl -s http://127.0.0.1:8010/v1/models
+systemctl restart vllm-gateway.service
+```
+
+## Rollback to systemd vLLM
+
+```bash
+cd /root/gpu-lab
+docker compose -f compose.vllm-legacy.yml down
+docker compose -f compose.vllm-modern.yml down
+docker compose -f compose.sglang.yml down
+systemctl start vllm.service
+systemctl restart vllm-gateway.service litellm-proxy.service
+```
+
+## Logs
+
+```bash
+docker logs -f gpu-lab-sglang
+journalctl -u model-manager -f
+```
+
+## SGLang on V100
+
+Always use `--disable-flashinfer` and `--disable-flashinfer-sampling` (in `compose.sglang.yml`).