fix: always update fingerprint stability; assert tracker fields preserved on skip

- Move fingerprint stability update before early return so it updates even when payload hash matches
- Remove duplicate stability assignment from detect_tracker result block
- Add assertion in test to verify tracker fields are preserved when detection is skipped

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_bluetooth_aggregator.py

@@ -568,10 +568,12 @@ class TestTrackerDetectionOptimization:
         with patch.object(
             aggregator._tracker_engine, "detect_tracker", wraps=aggregator._tracker_engine.detect_tracker
         ) as mock_detect:
-            aggregator.ingest(sample_observation)
+            device = aggregator.ingest(sample_observation)
             assert mock_detect.call_count == 0, (
                 "detect_tracker should not be called when the device payload fingerprint is unchanged"
             )
+            # Tracker fields from first ingest must be preserved on skip
+            assert device.payload_fingerprint_id is not None
 
     def test_tracker_detection_runs_when_payload_changes(self, aggregator, sample_observation):
         """detect_tracker must be called again when the device's manufacturer data changes."""

utils/bluetooth/aggregator.py

@@ -334,6 +334,9 @@ class DeviceAggregator:
         # Record sighting for persistence tracking.
         self._tracker_engine.record_sighting(fingerprint.fingerprint_id)
 
+        # Always update stability (can change as device fields are filled in).
+        device.payload_fingerprint_stability = fingerprint.stability_confidence
+
         # Only re-run the expensive signature scan when the payload has changed.
         if fingerprint.fingerprint_id == device.payload_fingerprint_id:
             return
@@ -356,7 +359,6 @@ class DeviceAggregator:
         device.tracker_confidence_score = result.confidence_score
         device.tracker_evidence = result.evidence
         device.payload_fingerprint_id = fingerprint.fingerprint_id
-        device.payload_fingerprint_stability = fingerprint.stability_confidence
 
     def _update_risk_analysis(self, device: BTDeviceAggregate) -> None:
         """Evaluate suspicious presence heuristics for a device."""