feat: add Test effect and native testing framework

- Add Test effect with operations: assert, assertEqual, assertNotEqual,
  assertTrue, assertFalse, fail
- Implement Test effect handlers in interpreter with TestResults tracking
- Add values_equal method for comparing Value types in tests
- Update lux test command to discover and run test_* functions
- Create example test files: test_math.lux, test_lists.lux
- Add TESTING_DESIGN.md documentation
- Fix AST mismatches in C backend and compiler.rs for compatibility

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/test_math.lux

@@ -0,0 +1,51 @@
+// Math Test Suite
+// Run with: lux test examples/test_math.lux
+
+fn test_addition(): Unit with {Test} = {
+    Test.assertEqual(4, 2 + 2)
+    Test.assertEqual(0, 0 + 0)
+    Test.assertEqual(100, 50 + 50)
+}
+
+fn test_subtraction(): Unit with {Test} = {
+    Test.assertEqual(0, 2 - 2)
+    Test.assertEqual(5, 10 - 5)
+    Test.assertEqual(-5, 0 - 5)
+}
+
+fn test_multiplication(): Unit with {Test} = {
+    Test.assertEqual(6, 2 * 3)
+    Test.assertEqual(0, 0 * 100)
+    Test.assertEqual(100, 10 * 10)
+}
+
+fn test_division(): Unit with {Test} = {
+    Test.assertEqual(2, 6 / 3)
+    Test.assertEqual(0, 0 / 5)
+    Test.assertEqual(5, 25 / 5)
+}
+
+fn test_comparisons(): Unit with {Test} = {
+    Test.assertTrue(5 > 3)
+    Test.assertTrue(3 < 5)
+    Test.assertTrue(5 >= 5)
+    Test.assertTrue(5 <= 5)
+    Test.assertFalse(3 > 5)
+    Test.assertFalse(5 < 3)
+}
+
+fn test_equality(): Unit with {Test} = {
+    Test.assertTrue(42 == 42)
+    Test.assertFalse(42 == 43)
+    Test.assertTrue(42 != 43)
+    Test.assertFalse(42 != 42)
+}
+
+fn test_boolean_logic(): Unit with {Test} = {
+    Test.assertTrue(true && true)
+    Test.assertFalse(true && false)
+    Test.assertTrue(true || false)
+    Test.assertFalse(false || false)
+    Test.assertTrue(!false)
+    Test.assertFalse(!true)
+}