feat: annotated video writer wraps OpenCV VideoWriter

tests/test_video_writer.py

@@ -0,0 +1,39 @@
+"""Tests for AnnotatedVideoWriter (src/video_writer.py)."""
+
+import cv2
+import numpy as np
+import pytest
+from src.video_writer import AnnotatedVideoWriter
+
+
+def test_writer_creates_output_file(tmp_path):
+    out = tmp_path / "test_output.mp4"
+    writer = AnnotatedVideoWriter(str(out), fps=25.0, frame_size=(640, 480))
+    frame = np.zeros((480, 640, 3), dtype=np.uint8)
+    writer.write_frame(frame)
+    writer.finish()
+    assert out.exists()
+    assert out.stat().st_size > 0
+
+
+def test_writer_multiple_frames(tmp_path):
+    out = tmp_path / "multi.mp4"
+    writer = AnnotatedVideoWriter(str(out), fps=25.0, frame_size=(320, 240))
+    for _ in range(10):
+        writer.write_frame(np.zeros((240, 320, 3), dtype=np.uint8))
+    writer.finish()
+    assert out.exists()
+
+
+def test_writer_close_idempotent(tmp_path):
+    out = tmp_path / "idem.mp4"
+    writer = AnnotatedVideoWriter(str(out), fps=25.0, frame_size=(320, 240))
+    writer.write_frame(np.zeros((240, 320, 3), dtype=np.uint8))
+    writer.finish()
+    writer.finish()  # second call should not raise
+    assert out.exists()
+
+
+def test_writer_invalid_fps():
+    with pytest.raises(ValueError):
+        AnnotatedVideoWriter("/tmp/x.mp4", fps=0.0, frame_size=(640, 480))