Problem:
Old 'autoread' only did `:checktime` on focus-change and shell (":!")
commands, and only for non-hidden buffers. Since 'autoread' is now
driven by OS filewatcher events, buffers are updated much more eagerly.
This should be surfaced to the user somehow, either via a carefully
placed notification, or a minimal UI indicator.
A "notification" would be noisy, unless it is conditional on specific
circumstances (e.g. when "many" buffers are updated).
Solution:
Use the existing 'busy' buffer-local option as a subtle hint about
activity.
Problem:
The 'autoread' option only checks for file changes reactively — on
FocusGained, :checktime, CmdlineEnter, etc. — by polling timestamps.
External changes are not detected until the user interacts with Neovim.
Solution:
Add a core module (runtime/lua/nvim/autoread.lua) enabled from
runtime/plugin/autoread.lua that watches each buffer's file using
vim._watch.watch() (libuv fs_event). On change detection it calls
:checktime, which invokes the existing buf_check_timestamp() logic
for reload/prompt handling. Watchers are managed via autocmds tied
to buffer lifecycle events and respect the 'autoread' option (global
and buffer-local).
