feat(autoread): use filewatchers for OS-driven change detection #37971

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).

runtime/plugin/autoread.lua

@@ -0,0 +1,12 @@
+-- File-watcher backing for the 'autoread' option.
+-- Lives here (not in vim/_core/defaults.lua) so that `-u NONE` / `--noplugin`
+-- skip it: the BufReadPost/BufWritePost autocmds it registers would otherwise
+-- show up in every test that inspects the autocmd list. Matches the pattern
+-- used by runtime/plugin/matchparen.lua.
+
+if vim.g.loaded_autoread ~= nil then
+  return
+end
+vim.g.loaded_autoread = 1
+
+require('nvim.autoread').enable()