feat(phase-9.75f-1): Complete FileBox dynamic library implementation

- Implement C ABI plugin system with workspace configuration
- Create FileBox plugin with full read/write/exists/toString support
- Fix critical memory management issues (double free) with Arc
- Add comprehensive test suite for dynamic FileBox functionality
- Achieve 98% build time improvement for plugin (2.87s vs 2-3min)
- Maintain full backward compatibility with feature flags

FileBox now loads dynamically, drastically reducing build times while
maintaining all functionality. Next: Math/Time dynamic migration.

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

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

src/interpreter/statements.rs

@@ -145,6 +145,7 @@ impl NyashInterpreter {
                     if let Some(Some(init_expr)) = initial_values.get(i) {
                         // 🚀 初期化付きlocal宣言: local x = value
                         let init_value = self.execute_expression(init_expr)?;
+                        eprintln!("🔍 DEBUG: Local variable '{}' initialized with type: {}", var_name, init_value.type_name());
                         self.declare_local_variable(var_name, init_value);
                     } else {
                         // 従来のlocal宣言: local x