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>

src/types.rs

@@ -1123,6 +1123,56 @@ impl TypeEnv {
             },
         );
 
+        // Add Test effect for test framework
+        env.effects.insert(
+            "Test".to_string(),
+            EffectDef {
+                name: "Test".to_string(),
+                type_params: Vec::new(),
+                operations: vec![
+                    EffectOpDef {
+                        name: "assert".to_string(),
+                        params: vec![
+                            ("condition".to_string(), Type::Bool),
+                            ("message".to_string(), Type::String),
+                        ],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "assertEqual".to_string(),
+                        params: vec![
+                            ("expected".to_string(), Type::Var(0)),
+                            ("actual".to_string(), Type::Var(0)),
+                        ],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "assertNotEqual".to_string(),
+                        params: vec![
+                            ("a".to_string(), Type::Var(0)),
+                            ("b".to_string(), Type::Var(0)),
+                        ],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "assertTrue".to_string(),
+                        params: vec![("condition".to_string(), Type::Bool)],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "assertFalse".to_string(),
+                        params: vec![("condition".to_string(), Type::Bool)],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "fail".to_string(),
+                        params: vec![("message".to_string(), Type::String)],
+                        return_type: Type::Unit,
+                    },
+                ],
+            },
+        );
+
         // Add Some and Ok, Err constructors
         // Some : fn(a) -> Option<a>
         let a = Type::var();