Replace the 1 s polling re-read of the browser directory with an inotify
watch (dirWatcher, internal/editor/dirwatcher.go) using golang.org/x/sys/unix,
which is already in the build list — no new dependency, no go.mod change.
- The watcher watches the current browser directory and coalesces a burst of
events to at most one token; the owner re-reads on the token. Create, delete,
rename, modify, and attrib (mtime) changes are all covered, so an inbound
sync that touches a file's mtime re-sorts the list in the current sort mode.
- The watch is re-pointed when the directory changes (a no-op while the path is
unchanged) and is closed when the logic goroutine exits (covers both Shutdown
and the test harness Done+WaitForExit path).
- A slow 30 s backstop rescan remains as a safety net for the rare event the
FUSE inotify layer drops (syncthing uses the same inotify+rescan pattern on
Android). The change-detection from the previous commit means the backstop
emits no frame when nothing changed, so TestNoFramesWhileIdle_Browser still
holds.
- If inotify is unavailable (newDirWatcher returns nil) or a watch add fails,
the 30 s backstop is the only path — a graceful degradation, not a break.
Verified on-device (Notes dir, Date-descending): an external add appears at the
top in well under a second (impossibly fast for the 30 s backstop, so the
inotify path); touching a file's mtime re-sorts it to the top; a delete drops
the row. All sub-second.
