feat: implement better error messages with 'Did you mean?' suggestions

Add Levenshtein distance-based similarity matching for undefined
variables, unknown types, unknown effects, and unknown traits.
When a name is not found, the error now suggests similar names
within edit distance 2.

Changes:
- Add levenshtein_distance() function to diagnostics module
- Add find_similar_names() and format_did_you_mean() helpers
- Update typechecker to suggest similar names for:
  - Undefined variables
  - Unknown types
  - Unknown effects
  - Unknown traits
- Add 17 new tests for similarity matching

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

src/main.rs

@@ -1221,6 +1221,21 @@ c")"#;
             assert!(diag.hints.iter().any(|h| h.contains("spelling")));
         }
 
+        #[test]
+        fn test_undefined_variable_suggestion() {
+            // Test that similar variable names are suggested
+            let source = r#"
+                let myVariable = 42
+                let x = myVriable
+            "#;
+            let result = super::eval(source);
+            assert!(result.is_err());
+            let err = result.unwrap_err();
+            // The error should contain a "Did you mean?" suggestion
+            assert!(err.contains("Did you mean") || err.contains("myVariable"),
+                "Error should suggest 'myVariable': {}", err);
+        }
+
         #[test]
         fn test_purity_violation_categorization() {
             let error = TypeError {