Function plan_append 

pub fn plan_append(
    last_pair_key: usize,
    prompt_len: usize,
    hidden_lo: usize,
    hidden_hi: usize,
) -> Option<AppendPlan>

2026-09-25: The warm-turn append after last_pair_key for a prompt of prompt_len tokens, reading hidden rows [hidden_lo, hidden_hi).

• first_key is last_pair_key + 1, raised to hidden_lo when the hidden store starts later; the skipped keys leave no hole (module docs).
• rows covers pair keys first_key ..= prompt_len - 2.

None when no key is missing, or when the store does not cover [first_key, prompt_len - 2].