Decide whether GRPO/GSPO need real old-logprob ratios

[adohe]

Spec candidate: #19.

Scope

• Write a short decision note comparing current surrogate semantics with real old-logprob ratio semantics.
• Include PPO as the existing reference for real old-logprob ratio behavior.
• Defer implementation unless maintainers approve a follow-up.

Acceptance

• Decision note has an explicit recommendation.
• Any behavior-changing follow-up is split into separate issues.