File_Watcher brought up to Ex4's (the pending index reset keeping its memory). Each changed .mcap is
looked at once settled: added when the grid lacks it, removed when its file is gone, read again when
it failed to load. A folder added, renamed or deleted, or lost events, compare the whole library as
Refresh does, quietly. Removals wait while the OCR thread is busy. viewer_add_capture returns the
index a capture already has, so an import or screen capture the watcher saw first is still scanned.
Off Windows, the deleted folder is no longer listed as part of the library.
