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>

test_scan_latest.py

@@ -0,0 +1,62 @@
+"""
+Tests for scan_latest's handling of unsniffable .MLG files.
+
+Regression guard: a .MLG file the watcher can't extract a unit ID from must
+be cached (as "no id") so it isn't re-sniffed and re-logged on every scan.
+Previously the unsniffable result was never cached, so the
+`[unsniffable-recent]` warning was re-emitted every scan cycle — ~200 files
+x 288 scans/day flooded the log with hundreds of thousands of duplicate lines.
+"""
+import os
+import time
+import tempfile
+import unittest
+
+import series3_watcher
+
+
+class UnsniffableCaching(unittest.TestCase):
+    def test_unsniffable_recent_file_logged_once_across_scans(self):
+        with tempfile.TemporaryDirectory() as d:
+            # A .MLG file with no BE####/BA#### pattern → sniff returns None.
+            path = os.path.join(d, "junk001.MLG")
+            with open(path, "wb") as f:
+                f.write(b"\x00" * 512)
+
+            calls = []
+            cache = {}
+            recent_cutoff = time.time() - 86400.0  # file (mtime≈now) counts as recent
+
+            for _ in range(3):
+                series3_watcher.scan_latest(
+                    watch=d,
+                    header_bytes=2048,
+                    cache=cache,
+                    recent_cutoff=recent_cutoff,
+                    max_age_days=365,
+                    logger=calls.append,
+                )
+
+            unsniff = [c for c in calls if "[unsniffable-recent]" in c]
+            self.assertEqual(
+                len(unsniff), 1,
+                "unsniffable file should log once across scans, got {}: {}".format(
+                    len(unsniff), unsniff),
+            )
+
+    def test_sniffable_file_still_returned(self):
+        # Sanity: a file WITH a unit id is still detected (happy path intact).
+        with tempfile.TemporaryDirectory() as d:
+            path = os.path.join(d, "evt001.MLG")
+            with open(path, "wb") as f:
+                f.write(b"hdr\x00BE12599\x00hdr")  # contains a sniffable unit id
+            cache = {}
+            result = series3_watcher.scan_latest(
+                watch=d, header_bytes=2048, cache=cache,
+                recent_cutoff=time.time() - 86400.0, max_age_days=365, logger=None,
+            )
+            self.assertIn("BE12599", result)
+
+
+if __name__ == "__main__":
+    unittest.main()