- drop `!arm64` from the build tags so linux/arm64 hits the real `fsnotify` watcher instead of the no-op fallback
- `inotify` is a kernel syscall and arch-independent; the 2016 carve-out has no technical basis
- without it, geth on linux/arm64 silently misses keystore changes and new keyfiles aren't picked up until restart
This PR removes some optimistic tests -- a'la "do something,
wait a while, and hope it has trickled through and continue" -- and
instead uses some introspection to ensure that prerequisites are met.
