feat: implement Schema module for versioned types

Add Schema module with functions for creating and migrating versioned
values. This provides the runtime foundation for schema evolution.

Schema module functions:
- Schema.versioned(typeName, version, value) - create versioned value
- Schema.migrate(value, targetVersion) - migrate to new version
- Schema.getVersion(value) - get version number

Changes:
- Add Versioned, Migrate, GetVersion builtins to interpreter
- Add Schema module to global environment
- Add Schema module type to type environment
- Add 4 tests for schema operations
- Add examples/versioning.lux demonstrating usage

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

src/main.rs

@@ -1333,6 +1333,50 @@ c")"#;
             assert!(err_msg.contains("outside") || err_msg.contains("Resume"),
                 "Error should mention resume outside handler: {}", err_msg);
         }
+
+        // Schema Evolution tests
+        #[test]
+        fn test_schema_versioned() {
+            let source = r#"
+                let user = Schema.versioned("User", 1, { name: "Alice", age: 30 })
+                let version = Schema.getVersion(user)
+            "#;
+            let (result, _) = run_with_effects(source, Value::Unit, Value::Unit).unwrap();
+            assert_eq!(result, "1");
+        }
+
+        #[test]
+        fn test_schema_migrate_same_version() {
+            let source = r#"
+                let user = Schema.versioned("User", 2, { name: "Bob" })
+                let migrated = Schema.migrate(user, 2)
+                let version = Schema.getVersion(migrated)
+            "#;
+            let (result, _) = run_with_effects(source, Value::Unit, Value::Unit).unwrap();
+            assert_eq!(result, "2");
+        }
+
+        #[test]
+        fn test_schema_migrate_upgrade() {
+            let source = r#"
+                let user = Schema.versioned("User", 1, { name: "Charlie" })
+                let migrated = Schema.migrate(user, 3)
+                let version = Schema.getVersion(migrated)
+            "#;
+            let (result, _) = run_with_effects(source, Value::Unit, Value::Unit).unwrap();
+            assert_eq!(result, "3");
+        }
+
+        #[test]
+        fn test_schema_migrate_downgrade_fails() {
+            let source = r#"
+                let user = Schema.versioned("User", 3, { name: "Dave" })
+                let migrated = Schema.migrate(user, 1)
+            "#;
+            let result = run_with_effects(source, Value::Unit, Value::Unit);
+            assert!(result.is_err());
+            assert!(result.unwrap_err().contains("downgrade"));
+        }
     }
 
     // Diagnostic rendering tests