feat(phase-9.75g-0): Implement Day 4 plugin system infrastructure (80% complete)

Core plugin system components implemented:
- nyash.toml parser for plugin configuration
- BoxFactoryRegistry for unified Box creation management
- PluginBox proxy for FFI boundary abstraction
- Runtime module integration

Key features:
- Simple TOML parsing without external dependencies
- Transparent Box switching (builtin ↔ plugin)
- Everything is Box philosophy maintained
- Thread-safe design with RwLock

Remaining Day 4 tasks:
- FileBox FFI interface definition
- Dynamic plugin loading with libloading
- Plugin FileBox integration tests

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

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

src/runtime/mod.rs

@@ -0,0 +1,11 @@
+//! Nyashランタイムモジュール
+//! 
+//! プラグインシステムとBox管理の中核
+
+pub mod plugin_config;
+pub mod box_registry;
+pub mod plugin_box;
+
+pub use plugin_config::PluginConfig;
+pub use box_registry::{BoxFactoryRegistry, BoxProvider, get_global_registry};
+pub use plugin_box::PluginBox;