Function runs_diff_page 

pub fn runs_diff_page(
    corpus: &str,
    service: &str,
    previous: Option<&str>,
    current: Option<&str>,
    pool: &State<DbPool>,
) -> Result<Template, Status>

The human diff-summary screen: the server-rendered status-transition matrix between two snapshots — the 1:1 HTML twin of api_run_diff, sharing RunDiffTransitionDto. A snapshot pair is chosen with two date dropdowns (a JS-free <form method=get>); each transition cell links into the runs_tasks_page drill-down pre-filtered to that previous → current transition. Reuses parse_snapshot_date, so a malformed date is a 400 and 404 on an unknown corpus/service. It replaced the legacy diff-summary binary route, which .unwrap()ed the date and panicked on the dispatch path (the F-1 gap, now closed).