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/unified_registry.rs

@@ -0,0 +1,49 @@
+/*!
+ * Global Unified Box Registry
+ * 
+ * Manages the global instance of UnifiedBoxRegistry
+ * Integrates all Box creation sources (builtin, user-defined, plugin)
+ */
+
+use crate::box_factory::{UnifiedBoxRegistry, builtin::BuiltinBoxFactory, plugin::PluginBoxFactory};
+use std::sync::{Arc, Mutex, OnceLock};
+
+/// Global registry instance
+static GLOBAL_REGISTRY: OnceLock<Arc<Mutex<UnifiedBoxRegistry>>> = OnceLock::new();
+
+/// Initialize the global unified registry
+pub fn init_global_unified_registry() {
+    GLOBAL_REGISTRY.get_or_init(|| {
+        let mut registry = UnifiedBoxRegistry::new();
+        
+        // Register built-in Box factory (highest priority)
+        registry.register(Arc::new(BuiltinBoxFactory::new()));
+        
+        // Register plugin Box factory (lowest priority)
+        registry.register(Arc::new(PluginBoxFactory::new()));
+        
+        // TODO: User-defined Box factory will be registered by interpreter
+        
+        Arc::new(Mutex::new(registry))
+    });
+}
+
+/// Get the global unified registry
+pub fn get_global_unified_registry() -> Arc<Mutex<UnifiedBoxRegistry>> {
+    init_global_unified_registry();
+    GLOBAL_REGISTRY.get().unwrap().clone()
+}
+
+/// Register a user-defined Box factory (called by interpreter)
+pub fn register_user_defined_factory(factory: Arc<dyn crate::box_factory::BoxFactory>) {
+    let registry = get_global_unified_registry();
+    let mut registry_lock = registry.lock().unwrap();
+    
+    // Insert at position 1 (after builtin, before plugin)
+    // This maintains priority: builtin > user > plugin
+    if registry_lock.factories.len() >= 2 {
+        registry_lock.factories.insert(1, factory);
+    } else {
+        registry_lock.register(factory);
+    }
+}