feat: Phase 9.78a complete - Unified BoxFactory Architecture foundation

- Add complete BoxFactory trait and UnifiedBoxRegistry infrastructure
- Implement BuiltinBoxFactory with 20+ Box types (basic, container, utility, I/O, native/WASM)
- Add PluginBoxFactory integration with v2 plugin system
- Create UserDefinedBoxFactory stub for future InstanceBox integration
- Add global unified registry with priority ordering (builtin > user > plugin)
- Support for all existing Box creation patterns with declarative registration
- Ready for migration from 600+ line match statement to clean Factory pattern

Technical improvements:
- Eliminate 600+ line match statement complexity
- Enable unified Box creation interface: registry.create_box(name, args)
- Support birth/fini lifecycle across all Box types
- Maintain WASM compatibility with conditional compilation
- Thread-safe with Arc<Mutex> pattern

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/runtime/mod.rs

@@ -5,6 +5,7 @@
 pub mod plugin_config;
 pub mod box_registry;
 pub mod plugin_loader_v2;
+pub mod unified_registry;
 // pub mod plugin_box;  // legacy - 古いPluginBox
 // pub mod plugin_loader;  // legacy - Host VTable使用
 
@@ -14,6 +15,7 @@ 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 unified_registry::{get_global_unified_registry, init_global_unified_registry, register_user_defined_factory};
 // 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