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).
