test(sveta): Write tests for variance function

Outcome: done
Changed: tests/test_calculator.py:2, tests/test_calculator.py:67-97
Verified by: make verify — exit 0, all 24 tests pass (8 variance tests OK)
Findings: Import line updated to include variance from src.calculator at tests/test_calculator.py:4
Findings: TestVarianceFunction class added at tests/test_calculator.py:67-97 with 8 test methods
Findings: Empty sequence coverage split into list and tuple cases at lines 69-76
Findings: Single element returns 0.0 at line 78
Findings: Constant values return 0.0 at line 81
Findings: Known set [1,2,3,4,5] yields variance 2.0 at line 84
Findings: Two-element edge case [0,2] yields variance 1.0 at line 87
Findings: Integer inputs produce float result at line 90
Findings: Tuple input accepted as Sequence at line 95
Findings: All patterns match existing test conventions — no comments, no docstrings, full type annotations, proper header
Open: none
Confidence: high — all acceptance criteria covered, pattern-consistent, verified pass

Typosaurus-Run: 918d38b1535b44bea9a86b82deae5233
Typosaurus-Node: 3f6d4a888da94bb99d585efb53106ebd
Typosaurus-Agent: @sveta
Refs: #19

tests/test_calculator.py

@@ -3,7 +3,7 @@
 import math
 import unittest
 
-from src.calculator import clamp
+from src.calculator import clamp, variance
 
 
 class TestClampFunction(unittest.TestCase):
@@ -61,3 +61,36 @@ class TestClampFunction(unittest.TestCase):
     def test_nan_returns_nan(self) -> None:
         result = clamp(math.nan, 0, 10)
         self.assertTrue(math.isnan(result))
+
+
+class TestVarianceFunction(unittest.TestCase):
+
+    def test_empty_list_raises_value_error(self) -> None:
+        with self.assertRaises(ValueError):
+            variance([])
+
+    def test_empty_tuple_raises_value_error(self) -> None:
+        with self.assertRaises(ValueError):
+            variance(())
+
+    def test_single_element_returns_zero(self) -> None:
+        self.assertEqual(variance([42.0]), 0.0)
+
+    def test_constant_values_return_zero_variance(self) -> None:
+        self.assertEqual(variance([1.0, 1.0, 1.0]), 0.0)
+
+    def test_population_variance_of_known_set(self) -> None:
+        self.assertEqual(variance([1, 2, 3, 4, 5]), 2.0)
+
+    def test_two_element_variance(self) -> None:
+        self.assertEqual(variance([0, 2]), 1.0)
+
+    def test_integer_inputs_return_float(self) -> None:
+        result = variance([10, 20, 30])
+        self.assertIsInstance(result, float)
+        self.assertEqual(result, 200.0 / 3.0)
+
+    def test_tuple_input_returns_variance(self) -> None:
+        self.assertEqual(variance((1, 2, 3, 4, 5)), 2.0)
+
+