chore: apply linting fixes and expand ruff coverage to test suite

tests/test_frame_stats.py

@@ -1,6 +1,8 @@
 import math
+
 import pytest
-from pygui.backend.models.frame_stats import compute_stats, Stats
+
+from pygui.backend.models.frame_stats import Stats, compute_stats
 
 
 def test_basic_stat():
@@ -20,3 +22,9 @@ def test_empty_values_returns_zeros():
 
 def test_ignores_nan():
     s = compute_stats([149.0, float("nan"), 151.0])
+    assert s.min == 149.0 and s.min_index == 0
+    assert s.max == 151.0 and s.max_index == 2
+    assert s.diff == pytest.approx(2.0)
+    assert s.avg == pytest.approx(150.0)
+    assert s.sigma == pytest.approx(1.0)
+    assert s.three_sigma == pytest.approx(3.0)