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>

examples/versioning.lux

@@ -0,0 +1,35 @@
+// Demonstrating Schema Evolution in Lux
+//
+// Lux provides versioned types to help manage data evolution over time.
+// The Schema module provides functions for creating and migrating versioned values.
+//
+// Expected output:
+// Created user v1: Alice (age unknown)
+// User version: 1
+// Migrated to v2: Alice (age unknown)
+// User version after migration: 2
+
+// Create a versioned User value at v1
+fn createUserV1(name: String): Unit with {Console} = {
+    let user = Schema.versioned("User", 1, { name: name })
+    let version = Schema.getVersion(user)
+    Console.print("Created user v1: " + name + " (age unknown)")
+    Console.print("User version: " + toString(version))
+}
+
+// Migrate a user to v2
+fn migrateUserToV2(name: String): Unit with {Console} = {
+    let userV1 = Schema.versioned("User", 1, { name: name })
+    let userV2 = Schema.migrate(userV1, 2)
+    let newVersion = Schema.getVersion(userV2)
+    Console.print("Migrated to v2: " + name + " (age unknown)")
+    Console.print("User version after migration: " + toString(newVersion))
+}
+
+// Main
+fn main(): Unit with {Console} = {
+    createUserV1("Alice")
+    migrateUserToV2("Alice")
+}
+
+let output = run main() with {}