Function write_watch_summary 

fn write_watch_summary(
    mount: &Path,
    is_system_drive: bool,
    values: &mut HashMap<String, f64>,
)

Summarise one already-collected volume as the watch target.

Reads the disk:vol:<mount>:* figures get_disk_data wrote rather than re-deriving them, which keeps the pill and the popover row arithmetically identical by construction. Pure w.r.t. the map, so the partial-tick behaviour below is testable.

A mount get_disk_data skipped (zero capacity, torn tick) has no figures to summarise: emit nothing rather than a fabricated 0, matching parseDiskVolumes, which drops partial volumes for the same reason.