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.

2 files changed, 141 insertions, 0 deletions

diff --git a/test/snapshots/filewatch.golden b/test/snapshots/filewatch.golden
new file mode 100644
index 00000000..64c43c85
--- /dev/null
+++ b/test/snapshots/filewatch.golden
@@ -0,0 +1,93 @@
+== snap edited grid=150x30 cursor=15,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/filewatch/cwd/w.txt Save De  NOR /tmp/pardes-snap/filewatch/cwd Del            NOR /tmp/pardes-snap/filewatch/cwd Del
+|     1 liMYEDITne 1
+|     2 line 2                                       w.txt                                             w.txt
+|     3 line 3
+|     4 line 4
+|     5 line 5
+|     6 line 6                                         w.txt
+|     7 line 7
+|     8 line 8
+|     9 line 9
+|    10 line 10
+|    11 line 11
+|    12 line 12
+|    13
+|
+|                                                    NOR /tmp/pardes-snap/filewatch/cwd Del
+|
+|                                                    w.txt
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+== snap reloaded grid=150x30 cursor=54,5
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/filewatch/cwd/w.txt Save De  TTY /tmp/pardes-snap/filewatch/cwd Del            NOR /tmp/pardes-snap/filewatch/cwd Del
+|     1 EXTERNAL                                     $ ls
+|     2                                              w.txt                                             w.txt
+|                                                    $ echo EXTERN''AL > w.txt
+|                                                    $
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|                                                    NOR /tmp/pardes-snap/filewatch/cwd Del
+|
+|                                                    w.txt
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+== snap undone grid=150x30 cursor=15,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/filewatch/cwd/w.txt Save De  TTY /tmp/pardes-snap/filewatch/cwd Del            NOR /tmp/pardes-snap/filewatch/cwd Del
+|     1 liMYEDITne 1                                 $ ls
+|     2 line 2                                       w.txt                                             w.txt
+|     3 line 3                                       $ echo EXTERN''AL > w.txt
+|     4 line 4                                       $
+|     5 line 5
+|     6 line 6
+|     7 line 7
+|     8 line 8
+|     9 line 9
+|    10 line 10
+|    11 line 11
+|    12 line 12
+|    13
+|
+|                                                    NOR /tmp/pardes-snap/filewatch/cwd Del
+|
+|                                                    w.txt
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
diff --git a/test/snapshots/filewatch.snap b/test/snapshots/filewatch.snap
new file mode 100644
index 00000000..1ac8d081
--- /dev/null
+++ b/test/snapshots/filewatch.snap
@@ -0,0 +1,48 @@
+# external file updates: a write from outside pardes lands in the open pane,
+# and Undo brings the unsaved edit back. That is the whole contract — the
+# update is committed like any other edit, so nothing has to be merged.
+lines w.txt 12
+start 30 150 -n 3
+wait 8000 w.txt
+stable 700 20000
+# open w.txt as a file pane: type the name into the shell, right-click it
+press left 5 8
+release left 5 8
+stable 400 5000
+key i
+text w.txt
+key esc
+settle 100
+press right 6 8
+release right 6 8
+wait 10000 line 3
+stable 700 15000
+# an UNSAVED edit on line 1
+press left 10 3
+release left 10 3
+stable 400 5000
+key i
+text MYEDIT
+key esc
+settle 100
+stable 400 5000
+snap edited
+# overwrite the file from a shell in another column. The marker is split so
+# the ECHOED command line never matches the wait — only the reloaded pane does
+press left 60 5
+release left 60 5
+stable 400 5000
+key c-b
+stable 600 8000
+text echo EXTERN''AL > w.txt
+key enter
+wait 10000 EXTERNAL
+stable 700 10000
+snap reloaded
+# back in the file pane: one undo and the unsaved edit is there again
+press left 10 3
+release left 10 3
+stable 400 5000
+key u
+stable 400 5000
+snap undone