fix: isolate per-camera tracker states in tensor-batched execution and add test_tracking

tests/test_tracking.py

@@ -0,0 +1,36 @@
+"""Tests for DetectionTracker multi-camera stream isolation."""
+
+from __future__ import annotations
+
+import unittest
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+from chicken_counter.config import load_camera_config
+
+
+class MultiCameraTrackerIsolationTests(unittest.TestCase):
+    def test_independent_stream_trackers(self) -> None:
+        cfg = load_camera_config("configs/cameras/example_camera.yaml")
+        
+        # Test get_or_create_tracker instantiates isolated instances for different stream IDs
+        with patch("chicken_counter.tracking.ensure_compatible_model", return_value=cfg.detection.model_path), \
+             patch("chicken_counter.tracking.YOLO"):
+            from chicken_counter.tracking import DetectionTracker
+
+            tracker_manager = DetectionTracker(cfg)
+            tr1 = tracker_manager.get_or_create_tracker("CC1")
+            tr2 = tracker_manager.get_or_create_tracker("CC2")
+
+            self.assertIsNotNone(tr1)
+            self.assertIsNotNone(tr2)
+            self.assertIsNot(tr1, tr2, "Trackers for CC1 and CC2 must be distinct objects")
+
+            # Resetting CC1 should not delete CC2 tracker
+            tracker_manager.reset_tracking("CC1")
+            self.assertNotIn("CC1", tracker_manager._stream_trackers)
+            self.assertIn("CC2", tracker_manager._stream_trackers)
+
+
+if __name__ == "__main__":
+    unittest.main()