feat(vllm): add LMCache sidecar in MP mode with period-notation flags

[genevera]

Adds an LMCache multiprocess (MP) sidecar container that runs alongside vLLM, following the official Docker quickstart.

Changes

• New compose.x.lmcache.vllm.yml: Defines the lmcache-server service using lmcache/standalone:nightly with the quickstart CLI flags (--l1-size-gb, --eviction-policy LRU, --max-workers, --port 6555), GPU device reservations for CUDA IPC, and shared memory (ipc: host, shm_size: 16g)
• profiles/default.env: Adds HARBOR_LMCACHE_L1_SIZE_GB, HARBOR_LMCACHE_MAX_WORKERS, HARBOR_LMCACHE_PORT; sets HARBOR_VLLM_EXTRA_ARGS to use period-notation --kv-transfer-config.* flags (appended, not replacing existing args)
• services/vllm/Dockerfile: Switches default base image to lmcache/vllm-openai (required for LMCacheMPConnector support in the vLLM container)
• New docs/2.1.VLLM-LMCache-Integration.md: Architecture and configuration docs

Key design decisions

• Period notation over JSON: --kv-transfer-config.kv_connector LMCacheMPConnector instead of inline JSON — easier to read and extend
• Compose network (not --network host): Host is tcp://lmcache-server via Docker Compose DNS, matching Harbor's harbor-network convention
• Append semantics: The kv-transfer-config flags are in HARBOR_VLLM_EXTRA_ARGS alongside any other flags; the compose overlay does not override the env var