Residual scroll jank at 120 Hz (suspected Slint event-loop bug) needs
per-frame + per-stage timing the logcat ring buffer can't hold. New
cargo feature `perf` (default OFF — all sites are cfg-gated, zero
overhead when off):
- FR: per-frame app-side render cost + frame gap, from the rendering
notifier (BeforeRendering/AfterRendering).
- STG: event-loop stage costs (wake/timers/events/poll/lastrender) from
timestamps instrumented into the pinned vendor copy of
i-slint-backend-android-activity (stg.rs; Cargo.toml now depends on
it directly + optional under `perf`).
- DECODE: per-thumbnail-job cost on the workers.
- All lines mirrored to /sdcard/Download/.gallery_perf.log (logcat
rotates too fast at ~120 lines/s).
Format + reading guide: docs/BUGS/scroll-jank-120hz.md.
