Function plan_rollover 

pub fn plan_rollover(
    messages: &[SharedMessage],
    budget: SuccessorBudget,
    before_tokens: u64,
    transcript_path: impl Fn(u64) -> PathBuf,
) -> Option<RolloverPlan>

Plan a rollover of messages into budget (message tokens, estimator units). transcript_path(window) names the file the evicted window is saved to. None when nothing can be evicted (no assistant round before the last one, or no valid cut); the caller treats that as no-shrink.

The successor is: the brief (message 0, always, even over budget); later orchestrator instructions from the evicted range, newest first within [PINNED_SHARE_PERCENT] of the budget; the host note; and the longest contiguous suffix that starts at an assistant message no tool group straddles and fits the rest of the budget. The last assistant message and everything after it are always kept.