feat(frontend): runtime-free assembly over a static worker file

[ishandhanani]

Stacked on #24. Roadmap Phase 4: frontend assembly without DistributedRuntime, model card independent of discovery, static-file feeder.

Summary

• StaticDirectEngine (lib/llm/src/kv_router/static_direct.rs): serves one model with no DistributedRuntime. Membership from a workers file (the selection service's --workers-file JSON shape; endpoint = the worker's direct gRPC endpoint), KV-aware selection on an embedded SelectionService that subscribes to the workers' ZMQ KV events itself (load-based routing when no worker publishes events), dispatch through the installed DirectEngineFactory. Bookings are freed when the response stream ends; prefill_complete is signalled on the first data chunk.
• entrypoint::input::http::run_static: HTTP service without a runtime. The builder assembly is shared (http_service_builder_for(.., Option<&DistributedRuntime>, ..)), so the runtime-backed path is unchanged.
• Python: make_static_engine(args, workers_file), run_static_input(engine, extensions), cancel_static_frontend(); dynamo.frontend --static-workers-file (DYN_FRONTEND_STATIC_WORKERS_FILE) takes the runtime-free path and never constructs DistributedRuntime. Requires --model-path (a local model directory; the card is read locally) and DYN_ROUTER_DIRECT_DISPATCH=vllm.
• WorkerRequest now derives Clone. Docs: frontend-configuration.mdx param field; sidecar-backends.md "Running Without A Distributed Runtime".

Validation

• cargo test -p dynamo-vllm-mocker static_direct: the mock vLLM gRPC server is named in a workers file (plus one worker without an endpoint), the assembly registers both, connects one, streams 3 tokens, and the booking is freed (free_reservation -> NotFound) after the stream ends. No runtime in the process.
• Live smoke with no etcd/NATS process running: two dynamo-vllm-mocker-server instances (ports 50061/50062) + python -m dynamo.frontend --static-workers-file ... --model-path <local Qwen3-0.6B> --model-name Qwen/Qwen3-0.6B --router-mode kv with DYN_ROUTER_DIRECT_DISPATCH=vllm: model served in 4 s, 4/4 chat completions OK (8 tokens each), log marker static direct frontend assembled without a distributed runtime workers=2 reachable=2, clean exit on SIGINT.
• cargo check --workspace --all-targets, cargo clippy --workspace --all-targets, bindings cargo check: clean. docs_lint.py: 0 errors.

Limitations: membership is fixed for the process lifetime; aggregated only (disaggregated, encoders, and worker churn use the runtime-backed frontend). Dependency on dynamo-runtime remains at compile time (pipeline types); removing it is Phase 5.

[ishandhanani]

Closed while Stack 1 (#45 to #53) and Stack 2 (#40) go upstream; the branch stays for later stacks (direct dispatch, catalog leases, runtime features). See #33.