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/lexer.rs

@@ -484,13 +484,21 @@ impl<'a> Lexer<'a> {
                             current_literal.push('}');
                         }
                         _ => {
+                            let escape_start = self.pos;
                             let escaped = match self.advance() {
                                 Some('n') => '\n',
                                 Some('r') => '\r',
                                 Some('t') => '\t',
                                 Some('\\') => '\\',
                                 Some('"') => '"',
-                                Some(c) => c,
+                                Some('0') => '\0',
+                                Some('\'') => '\'',
+                                Some(c) => {
+                                    return Err(LexError {
+                                        message: format!("Invalid escape sequence: \\{}", c),
+                                        span: Span::new(escape_start - 1, self.pos),
+                                    });
+                                }
                                 None => {
                                     return Err(LexError {
                                         message: "Unterminated string".into(),