Function window_start_rev 

pub fn window_start_rev(db: &FactsDb, wd: u32) -> Result<Option<String>>

The revision representing a file’s state entering the trailing window: the newest commit strictly before the window opened. None when the window spans all history (no pre-window commit) — every file is then judged as newly-added, which is also the “history shallower than the window” signal the [new_code] gate skips on.

Shared with the [new_code] gate so both views anchor the window start on the same rev; the caller passes the window length (the effort view uses opts.window_days, the new-code gate its own [new_code] window_days).