Design the minimal vLLM integration boundary

[DaBestCode]

Define how RandKV should integrate with vLLM before coupling the policy to a fast-moving internal API. The output is a reviewed design plus the smallest testable prototype.

Acceptance criteria:

• Pin and document the target vLLM version/commit.
• Map prompt protection, recent-buffer protection, deterministic request identity, eviction cadence, and observability onto vLLM concepts.
• Explain whether selection can remain independent per KV head when vLLM allocates paged blocks; call out any semantic compromise explicitly.
• Identify the narrow public or internal hook and its upgrade risk.
• Include call-site-first configuration requiring no more than one RandKV option.
• Add CPU-testable policy/adapter boundary tests before requiring CUDA.
• Document cancellation, prefix caching, chunked prefill, and multi-request behavior.

Start with a design document or draft PR; do not optimize kernels in this issue.