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_watcher.py

@@ -237,13 +237,16 @@ def scan_latest(
                     uid = cached[1]
                 else:
                     uid = sniff_unit_from_mlg(fpath, header_bytes)
-                    if not uid:
-                        # If unsniffable but very recent, log for later inspection
-                        if (recent_cutoff is not None) and (mtime >= recent_cutoff):
-                            if logger:
-                                logger("[unsniffable-recent] {}".format(fpath))
-                        continue  # skip file if no unit ID found in header
+                    # Cache the result either way — including unsniffable (uid=None) —
+                    # so the same file isn't re-sniffed and re-logged on every scan.
                     cache[fpath] = (mtime, uid)
+                    if (not uid) and (recent_cutoff is not None) and (mtime >= recent_cutoff):
+                        # Log once, on first sight, for later inspection.
+                        if logger:
+                            logger("[unsniffable-recent] {}".format(fpath))
+
+                if not uid:
+                    continue  # no unit ID in header — skip (cached above, won't re-log)
 
                 if (uid not in latest) or (mtime > latest[uid]["mtime"]):
                     latest[uid] = {"mtime": mtime, "fname": e.name, "path": fpath}
@@ -253,7 +256,7 @@ def scan_latest(
 
 
 # --- API heartbeat / SFM telemetry helpers ---
-VERSION = "1.6.0"
+VERSION = "1.6.1"
 
 
 def _read_log_tail(log_file: str, n: int = 25) -> Optional[list]: