- Trace reads /proc/stat btime and /proc/uptime at import so every
event in /api/diag/timing can be expressed as 'seconds since
power-on' (uptime_at_anchor + ev.t).
- Web.StaticFileHandler.prepare emits 'web.first_root_get' the first
time chromium hits / or /index.html, so we can see when the kiosk
browser actually started loading the UI on cold boot.