[WIP] Fix Opal runtime compatibility for adapter loading

[suleman-uzair]

Summary

Adds runtime compatibility handling so adapter, schema, and XML serialization loading work correctly under Opal.

Changes

• Adds shared runtime helpers for Opal/native checks, native-only requires/autoloads, and safe constant lookup.
• Uses Opal-safe load paths for key-value and XML adapters.
• Avoids native-only TOML adapter defaults in Opal.
• Ensures XML serialization/import methods are applied directly to Serializable under Opal.
• Keeps Nokogiri-specific entity handling limited to native XML parsing paths.