feat: Implement nyash.toml v2 plugin system with PluginLoaderV2

- Add plugin_loader_v2.rs with nyash.toml v2 support
- Create PluginBoxV2 as temporary wrapper for v2 plugins
- Update runner.rs to use v2 loader instead of legacy BID registry
- Update box_registry.rs to work with v2 plugin system
- Support single FFI entry point (nyash_plugin_invoke) + optional init
- Integrate with existing BoxFactoryRegistry for seamless Box creation
- All compilation errors resolved, ready for testing

src/runtime/mod.rs

@@ -4,6 +4,7 @@
 
 pub mod plugin_config;
 pub mod box_registry;
+pub mod plugin_loader_v2;
 // pub mod plugin_box;  // legacy - 古いPluginBox
 // pub mod plugin_loader;  // legacy - Host VTable使用
 
@@ -12,6 +13,7 @@ mod tests;
 
 pub use plugin_config::PluginConfig;
 pub use box_registry::{BoxFactoryRegistry, BoxProvider, get_global_registry};
+pub use plugin_loader_v2::{PluginLoaderV2, get_global_loader_v2, init_global_loader_v2};
 // pub use plugin_box::PluginBox;  // legacy
 // Use unified plugin loader (formerly v2)
 // pub use plugin_loader::{PluginLoaderV2 as PluginLoader, get_global_loader_v2 as get_global_loader};  // legacy