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/config/nyash_toml_v2.rs

@@ -61,6 +61,9 @@ pub struct MethodDefinition {
     /// Optional argument declarations (v2.1+)
     #[serde(default)]
     pub args: Option<Vec<ArgDecl>>, 
+    /// Optional: wrap return and errors into ResultBox when true
+    #[serde(default)]
+    pub returns_result: bool,
 }
 
 /// Method argument declaration (v2.1+)