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/types.rs

@@ -1115,6 +1115,29 @@ impl TypeEnv {
             TypeScheme::mono(Type::function(vec![Type::var()], Type::String)),
         );
 
+        // Schema module for versioned types
+        let schema_module_type = Type::Record(vec![
+            (
+                "versioned".to_string(),
+                Type::function(
+                    vec![Type::String, Type::Int, Type::var()],
+                    Type::var(), // Returns Versioned (treated as Any for now)
+                ),
+            ),
+            (
+                "migrate".to_string(),
+                Type::function(
+                    vec![Type::var(), Type::Int],
+                    Type::var(), // Returns Versioned
+                ),
+            ),
+            (
+                "getVersion".to_string(),
+                Type::function(vec![Type::var()], Type::Int),
+            ),
+        ]);
+        env.bind("Schema", TypeScheme::mono(schema_module_type));
+
         env
     }