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/lib.rs

@@ -32,6 +32,9 @@ pub mod value;
 // 🚀 BID-FFI: Box Interface Definition with FFI (NEW!)
 pub mod bid;
 
+// 🔌 Runtime: Plugin System and Box Management (NEW!)
+pub mod runtime;
+
 // 🌐 P2P Communication Infrastructure (NEW!)
 pub mod messaging;
 pub mod transport;