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/parser.rs

@@ -478,11 +478,11 @@ impl Parser {
             self.advance();
             self.skip_newlines();
 
-            if self.check(TokenKind::Pipe) || self.peek_is_variant() {
-                // Enum type
+            if self.check(TokenKind::Pipe) {
+                // Enum type - requires leading | for variants
                 (TypeDef::Enum(self.parse_variants()?), Vec::new())
             } else {
-                // Type alias
+                // Type alias - any type expression
                 (TypeDef::Alias(self.parse_type()?), Vec::new())
             }
         } else {