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>

README.md

@@ -1,4 +1,4 @@
-# Series 3 Watcher v1.6.0
+# Series 3 Watcher v1.6.1
 
 Monitors Instantel **Series 3 (Minimate)** call-in activity on a Blastware server. Runs as a **system tray app** that starts automatically on login, reports heartbeats to terra-view, and self-updates from Gitea.
 
@@ -188,7 +188,7 @@ where the corresponding server-side work lives.
 
 ## Versioning
 
-Follows **Semantic Versioning**. Current release: **v1.6.0**.
+Follows **Semantic Versioning**. Current release: **v1.6.1**.
 See `CHANGELOG.md` for full history.
 
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