feat: Implement returns_result and improve HTTP plugin stability

- Add returns_result=true configuration for HTTP server methods
- Fix ResultBox downcasting in interpreter calls
- Improve HTTP plugin with server_id tracking and response ID hints
- Add TCP connection support with fallback to stub mode
- Implement response ID mapping improvements
- Fix various race conditions in concurrent request handling

This partially addresses the HTTP response body empty issue, though
ID mapping still needs further refinement.

Co-Authored-By: ChatGPT5 <noreply@openai.com>
Co-Authored-By: Claude <noreply@anthropic.com>

src/interpreter/expressions/calls.rs

@@ -270,7 +270,7 @@ impl NyashInterpreter {
         */
         
         // ResultBox method calls
-        if let Some(result_box) = obj_value.as_any().downcast_ref::<crate::box_trait::ResultBox>() {
+        if let Some(result_box) = obj_value.as_any().downcast_ref::<crate::boxes::ResultBox>() {
             return self.execute_result_method(result_box, method, arguments);
         }