open files follow the disk, and undo is the merge strategy

An external update is pushed onto the undo stack exactly like an edit the user
typed, so unsaved work is one `u` away and pardes never has to merge anything.
That is the design, not an implementation detail: the whole feature is
pushUndo() then setContent().

One inotify instance in the tty shell, blocking in readVec through std.Io on a
concurrent task started beside the pty readers — after loop.start(), so the
forkpty ordering is untouched. It watches the containing DIRECTORY, because an
editor rewrites by rename-over and a watch on the file would follow the dead
inode, and it listens for CLOSE_WRITE rather than MODIFY, which is one event per
finished writer and most of the debounce for free.

Our own Save does not reach the undo stack: each watch keeps a hash of the
bytes last seen on disk and save_file restamps it. A hash rather than mtime and
size because the reload has to read the file anyway.

The core stays sans-IO — one watch effect out, one file_changed event in, and a
shell that cannot watch simply never sends the event, which is what the gui and
the web platform do. Linux only; fanotify is what the build system uses and is
rejected in a comment: it exists for thousands of directories across mounts,
and sixteen panes of inotify is a third of the code with no kernel floor.

New golden filewatch: edit without saving, overwrite from a shell in another
column, watch it reload, undo, get the unsaved edit back. None moved.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/gui/gui.zig b/src/gui/gui.zig
index d8086981..998573b5 100644
--- a/src/gui/gui.zig
+++ b/src/gui/gui.zig
@@ -2137,6 +2137,11 @@ fn drainEffects(
             _ = c.SDL_SetClipboardText(z.ptr);
         },
         .lsp => |e| if (threads_ok) spawnLsp(core, gpa, queue, e),
+        // ponytail: the SDL shell does not watch files. Everything the core
+        // needs is already here (the effect and the file_changed event) — what
+        // is missing is the ~50 lines of inotify plumbing in tty.zig, and a
+        // shell that never delivers the event simply never reloads.
+        .watch => {},
         .quit => {},
     };
 }
@@ -2174,8 +2179,9 @@ fn drainEffectsWeb(core: *pardes.Pardes, gpa: std.mem.Allocator, g: *Gui) void {
         },
         // look on a URL → a new tab
         .open_link => |url| openLinkWeb(gpa, url.slice()),
-        // nothing to spawn/write/resize/save/dump into — all no-ops
-        .spawn, .write, .resize_pty, .save_file, .write_dump, .lsp, .quit => {},
+        // nothing to spawn/write/resize/save/dump into, and no filesystem to
+        // watch — all no-ops
+        .spawn, .write, .resize_pty, .save_file, .write_dump, .lsp, .watch, .quit => {},
     };
 }