feat: Add parallel HTTP server E2E tests and enhance plugin system

- Add e2e_plugin_net_additional.rs with parallel server tests
- Fix test to properly handle request objects (no double accept)
- Add comprehensive net-plugin documentation
- Implement debug tracing for method calls
- Enhance plugin lifecycle documentation
- Improve error handling in plugin loader
- Add leak tracking infrastructure (for future use)
- Update language spec with latest plugin features

This enhances test coverage for concurrent HTTP servers and improves
the overall plugin system documentation and debugging capabilities.

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

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

src/interpreter/expressions/calls.rs

@@ -996,6 +996,10 @@ impl NyashInterpreter {
         method: &str,
         arguments: &[ASTNode],
     ) -> Result<Box<dyn NyashBox>, RuntimeError> {
+        // Guard: use-after-fini is a runtime error (明示ライフサイクル)
+        if plugin_box.is_finalized() {
+            return Err(RuntimeError::RuntimeFailure { message: format!("Use after fini: {}", plugin_box.box_type) });
+        }
         eprintln!("🔍 execute_plugin_box_v2_method called: {}.{}", plugin_box.box_type, method);
         let mut arg_values: Vec<Box<dyn NyashBox>> = Vec::new();
         for arg in arguments {