fix: updater survives Settings saves + cache unsniffable MLGs (v1.6.1)

Two reliability fixes for the Series 3 tray app:

1. Auto-updater no longer dies on a Settings save. _restart_watcher
   set+replaced the shared stop_event that the updater loop also keyed
   off, so saving settings could kill the updater thread (race). Give
   the updater its own app-lifetime event (_app_stop). Same fix as
   thor-watcher 0.4.1.

2. Unsniffable .MLG files are now cached, so each is sniffed + logged
   once per session instead of every 5-min scan. A long-lived autocall
   folder with a few hundred unidentifiable files was flooding the log
   (~600K lines in 10 days) and re-reading every header each scan.

Adds test_series3_tray.py (4 lifecycle tests) and test_scan_latest.py
(2 scan tests). Full suite 50 passing. Bump to v1.6.1.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

settings_dialog.py

@@ -1,5 +1,5 @@
 """
-Series 3 Watcher — Settings Dialog  v1.6.0
+Series 3 Watcher — Settings Dialog  v1.6.1
 
 Provides a Tkinter settings dialog that doubles as a first-run wizard.