Function plan_review_diff 

pub fn plan_review_diff(
    last_reviewed: Option<&str>,
    head: Option<&str>,
    still_on_branch: bool,
) -> ReviewDiff

Decide what a review should show (DESIGN.md §8). last_reviewed is the revision recorded when the operator rejected, head the branch’s current tip, and still_on_branch whether that recorded revision is still reachable from the head — false after a force-push or a rebase rewrote it away.

Every gap degrades to the full diff rather than erroring: a task nobody has rejected yet reviews exactly as it always did (no notice, so nothing about a first review changes), and a delta that cannot be built says why.