fix: resolve all stress test bugs

- Record equality: add Record case to values_equal in interpreter
- Invalid escapes: error on unknown escape sequences in lexer
- Unknown effects: validate effect names in check_function with suggestions
- Circular types: add DFS cycle detection in check_type_cycles
- Parser: require | for enum variants, enabling proper type alias syntax

All 265 tests pass.

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

src/main.rs

@@ -1813,6 +1813,46 @@ c")"#;
         assert_eq!(eval(r#"let x = typeOf("hello")"#).unwrap(), r#""String""#);
     }
 
+    // Bug fix tests
+    #[test]
+    fn test_record_equality() {
+        assert_eq!(eval("let x = { a: 1, b: 2 } == { a: 1, b: 2 }").unwrap(), "true");
+        assert_eq!(eval("let x = { a: 1 } == { a: 2 }").unwrap(), "false");
+        assert_eq!(eval("let x = { a: 1, b: 2 } == { a: 1, b: 3 }").unwrap(), "false");
+    }
+
+    #[test]
+    fn test_invalid_escape_sequence() {
+        let result = eval(r#"let x = "\z""#);
+        assert!(result.is_err());
+        assert!(result.unwrap_err().contains("Invalid escape sequence"));
+    }
+
+    #[test]
+    fn test_unknown_effect_error() {
+        let result = eval("fn test(): Unit with {FakeEffect} = ()");
+        assert!(result.is_err());
+        assert!(result.unwrap_err().contains("Unknown effect"));
+    }
+
+    #[test]
+    fn test_circular_type_definitions() {
+        let result = eval("type A = B\ntype B = A");
+        assert!(result.is_err(), "Should detect circular type definitions");
+        let err = result.unwrap_err();
+        assert!(err.contains("Circular"), "Error should mention 'Circular': {}", err);
+    }
+
+    #[test]
+    fn test_mutual_recursion() {
+        let source = r#"
+            fn isEven(n: Int): Bool = if n == 0 then true else isOdd(n - 1)
+            fn isOdd(n: Int): Bool = if n == 0 then false else isEven(n - 1)
+            let result = isEven(10)
+        "#;
+        assert_eq!(eval(source).unwrap(), "true");
+    }
+
     // Pipe with stdlib tests
     #[test]
     fn test_pipe_with_list() {
@@ -2948,7 +2988,6 @@ c")"#;
     mod example_tests {
         use super::*;
         use std::fs;
-        use std::path::Path;
 
         fn check_file(path: &str) -> Result<(), String> {
             let source = fs::read_to_string(path)