kqueue's NOTE_WRITE on a directory fires only when directory entries are
added or removed, not when file contents change. This meant writes to
existing files were never reported as 'modified' events on macOS/FreeBSD.
Fix by maintaining a second set of kqueue watches on individual files
(file_watches), registered with NOTE_WRITE|NOTE_EXTEND. When either flag
fires on a file fd, a 'modified' event is emitted. File watches are
registered in take_snapshot (for files already present when watch() is
called) and in scan_dir (for newly created files), and deregistered when
files are deleted or the directory is unwatched.
Also fix two related bugs:
- NOTE_DELETE was incorrectly defined as 0x04 (NOTE_EXTEND); the correct
value is 0x01. This could cause NOTE_EXTEND events on watched directories
to be misreported as directory-deleted events.
- scan_dir emitted created events before deleted events, so a rename
(old name disappears, new name appears) reported the destination before
the source. Swapped the order so deletions are always emitted first.
Simplify thread_fn/arm to pass *KQueueBackend directly now that the backend
lives at a stable heap address inside the heap-allocated Interceptor.
All 10 integration tests now pass on FreeBSD.
Handler callbacks invoked while holding snapshots_mutex could deadlock if
the handler called watch() or unwatch(), which also acquires that lock.
Refactor scan_dir to collect all pending events (dir_created, created,
deleted) into temporary lists under the lock, then emit them after
releasing it. Also consolidate the two directory iteration passes into one.
