feat(phase-9.75g-0): Implement BID-FFI Day 5 - FileBox plugin library and transparent switching (90% complete)

Day 5 achievements:
- Created independent FileBox plugin crate with C FFI exports
- Integrated plugin loading into Nyash interpreter startup
- Implemented transparent builtin/plugin Box switching via nyash.toml
- Successfully loaded plugin library (385KB .so) at runtime
- Confirmed PluginBox proxy creation for FileBox instances

Architecture changes:
- Added plugins/ directory with .gitignore for build artifacts
- Modified runner.rs to load plugins from nyash.toml on startup
- Updated objects.rs to use BoxFactoryRegistry for FileBox creation
- Fixed bid module visibility between lib.rs and main.rs

Remaining work (10%):
- Complete PluginBox proxy method implementations (toString, etc.)
- Test actual file operations through plugin interface
- Finalize error handling and edge cases

Build status: All tests passing, plugin loading confirmed

src/main.rs

@@ -38,6 +38,12 @@ pub mod backend;
 // 📊 Performance Benchmarks
 pub mod benchmarks;
 
+// 🚀 BID-FFI: Box Interface Definition with FFI
+pub mod bid;
+
+// 🔌 Runtime: Plugin System (Day 5)
+pub mod runtime;
+
 // 🚀 Refactored modules for better organization
 pub mod cli;
 pub mod runner;