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/typechecker.rs

@@ -1532,7 +1532,7 @@ impl TypeChecker {
         }
 
         // Built-in effects are always available
-        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process", "Http", "HttpServer"];
+        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process", "Http", "HttpServer", "Test"];
         let is_builtin = builtin_effects.contains(&effect.name.as_str());
 
         // Track this effect for inference