Define rollout-engine vocabulary for future work

[adohe]

Spec candidate: #10.

Scope

• Define what AReno means by a future RolloutEngine.
• Define token-in/token-out and future trajectory metadata expectations.
• Define expected operations conceptually: generate, begin/end session, sleep/wake if supported, and load/sync weights if supported.

Acceptance

• Conceptual only; no external runtime backend integration is claimed.
• Explains how this differs from the existing top-level Backend ABC.
• Explicitly says v0.0.3 is names/docs/contracts, not external backend integration.