docs: define vime integration design

[inaniloquentee]

Summary

• Add the WS5 RL-Kernel to Vime integration design doc for [WS5] RL-Kernel to vime integration design doc #119.
• Define the non-intrusive activation model, ownership boundaries, lifecycle, tensor/API contracts, and rollout/training-side responsibilities.
• Capture the change-surface estimate across RL-Kernel, Vime/slime adapter work, and upstream vLLM/vllm-router requirements.
• Document risk/fallback behavior and the recommended [WS5] Minimal vime PoC with one RL-Kernel operator #120 PoC path.

Scope

This is a documentation-only PR.

It does not add a production integration, change Vime/slime/vLLM/Megatron source, inject RL-Kernel operators, or claim real-engine train-inference consistency.

Depends on #118
Closes #119
Part of #83

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