Add the first AReno failure-mode table

[adohe]

Spec candidate: #6.

Scope

• Add a small failure-mode table for one high-risk path.
• Recommended first table: GRPO/GSPO ratio-vs-drift confusion, rollout/train logprob mismatch, or agent function/tool-result trajectory bugs.
• Use the pith-train pattern: symptom -> first thing to inspect -> likely fix or next verification.

Acceptance

• Table is concrete and points to real files/tests.
• Does not claim all failure modes are covered.
• No behavior changes.