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>

series3_tray.py

@@ -1,5 +1,5 @@
 """
-Series 3 Watcher — System Tray Launcher  v1.6.0
+Series 3 Watcher — System Tray Launcher  v1.6.1
 Requires: pystray, Pillow, tkinter (stdlib)
 
 Run with:  pythonw series3_tray.py   (no console window)
@@ -335,7 +335,8 @@ def _show_cancel_message():
 class WatcherTray:
     def __init__(self):
         self.state = {}
-        self.stop_event = threading.Event()
+        self.stop_event = threading.Event()   # watcher lifecycle; replaced on restart
+        self._app_stop = threading.Event()    # app lifetime; only set on exit/update
         self._watcher_thread = None
         self._icon = None
         # Lock guards _rebuild_menu calls from the updater thread
@@ -394,6 +395,7 @@ class WatcherTray:
                 subprocess.Popen(["explorer", HERE])
 
     def _exit(self, icon, item):
+        self._app_stop.set()
         self.stop_event.set()
         icon.stop()
 
@@ -459,7 +461,7 @@ class WatcherTray:
         last_status = None
         update_check_counter = 0  # check for updates every ~5 min (30 * 10s ticks)
 
-        while not self.stop_event.is_set():
+        while not self._app_stop.is_set():
             icon_status = self._tray_status()
 
             if self._icon is not None:
@@ -487,7 +489,7 @@ class WatcherTray:
                     self._do_update(url)
                     return  # exit loop; swap bat will relaunch
 
-            self.stop_event.wait(timeout=10)
+            self._app_stop.wait(timeout=10)
 
     def _do_update(self, download_url=None):
         """Notify tray icon then apply update. If url is None, fetch it first."""
@@ -502,6 +504,7 @@ class WatcherTray:
 
         success = apply_update(download_url)
         if success:
+            self._app_stop.set()
             self.stop_event.set()
             if self._icon is not None:
                 self._icon.stop()