Add portable vLLM token artifacts for Responses continuations

Parent tracker: #69
Design and measured evidence: #65

Summary

Add portable, validated token artifacts for Responses continuations after exact llm-d routing and reliable checkpoint persistence are established.

The artifact path is a transport/rendering optimization. It must preserve the KV-cache hit ratio achieved by exact routing and must not depend on a process-local handle that is valid only before llm-d selects a replica.

Scope

• Accept a private portable artifact reference or certified prompt tokens in vLLM and feed the resolved complete token sequence into the existing input/APC path.
• Replace unbounded process-local prompt dictionaries with a bounded, tenant-scoped, expiring local cache backed by an authenticated portable artifact service or inline reseed path.
• Bind artifacts and append certificates to model, tokenizer, renderer/template/parser, instructions, tools, cache_salt, LoRA, multimodal identity, and expiration.
• Define typed artifact miss, expiry, validation failure, and reseed behavior with safe fallback to ordinary full Responses rendering.
• Accumulate all generated streaming token IDs and preserve tool-round boundaries before optionally advancing an artifact.
• Coordinate llm-d placement with authenticated router receipts or server-owned artifact resolution; endpoint-local shortcuts may only be applied after endpoint selection.
• Avoid returning full prompt-token arrays to public clients or creating a second KV-cache namespace outside vLLM APC.

Acceptance criteria

• Any selected compatible replica can resolve the artifact or safely reseed from authenticated material.
• Artifact miss, expiry, restart, wrong-pod selection, and validation failure deterministically fall back without changing model-visible token IDs.
• Portable artifact execution produces the same vLLM APC lookup identity as ordinary full rendering.
• Cache-hit ratio remains equal to the exact-routing baseline while long-context request transfer, reconstruction, rendering/tokenization work, or TTFT improves.
• Local artifact storage is bounded, expiring, tenant-isolated, observable, and safe under process rotation.

Start with parent tracker #69 and the design and measured evidence in #65. Map the vLLM APC path, llm-d placement, artifact validation, reseeding, and streaming token boundaries before proposing an implementation. Done means compatible replicas resolve or safely reseed artifacts, fallbacks preserve token IDs and APC identity, and bounded tenant-scoped storage meets the listed acceptance criteria.