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/runtime/mod.rs

@@ -5,6 +5,7 @@
 pub mod plugin_config;
 pub mod box_registry;
 pub mod plugin_loader_v2;
+pub mod leak_tracker;
 pub mod unified_registry;
 pub mod nyash_runtime;
 // pub mod plugin_box;  // legacy - 古いPluginBox