feat: Implement plugin singleton pattern with shutdown support

- Add singleton support for plugin boxes (e.g., CounterBox)
- Implement shutdown_plugins_v2() for controlled plugin lifecycle
- Plugin instances now shared across multiple new() calls
- Shutdown properly releases and allows re-initialization
- All singleton E2E tests passing ✅

ChatGPT5による高度なプラグインライフサイクル管理実装
- シングルトンパターンでプラグインインスタンス共有
- 明示的なshutdownでリソース解放と再初期化対応
- Nyashの統一ライフサイクルポリシー維持

Note: ast.rs test failures are due to rapid development pace -
tests need updating for new BoxDeclaration fields (private_fields, public_fields)

🤖 Generated with Claude Code

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

src/config/nyash_toml_v2.rs

@@ -47,6 +47,10 @@ pub struct BoxTypeConfig {
     
     /// Method definitions
     pub methods: HashMap<String, MethodDefinition>,
+
+    /// Singleton service flag (keep one shared instance alive in loader)
+    #[serde(default)]
+    pub singleton: bool,
 }
 
 /// Method definition (simplified - no argument info needed)