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/debug/leak_tracker.rs

@@ -0,0 +1,38 @@
+use once_cell::sync::Lazy;
+use std::collections::HashMap;
+use std::sync::Mutex;
+
+static ENABLED: Lazy<bool> = Lazy::new(|| std::env::var("NYASH_LEAK_LOG").unwrap_or_default() == "1");
+static LEAKS: Lazy<Mutex<HashMap<(String, u32), &'static str>>> = Lazy::new(|| Mutex::new(HashMap::new()));
+
+pub fn init() {
+    let _ = &*REPORTER; // ensure reporter is constructed
+}
+
+pub fn register_plugin(box_type: &str, instance_id: u32) {
+    if !*ENABLED { return; }
+    let mut m = LEAKS.lock().unwrap();
+    m.insert((box_type.to_string(), instance_id), "plugin");
+}
+
+pub fn finalize_plugin(box_type: &str, instance_id: u32) {
+    if !*ENABLED { return; }
+    let mut m = LEAKS.lock().unwrap();
+    m.remove(&(box_type.to_string(), instance_id));
+}
+
+struct Reporter;
+impl Drop for Reporter {
+    fn drop(&mut self) {
+        if !*ENABLED { return; }
+        let m = LEAKS.lock().unwrap();
+        if m.is_empty() { return; }
+        eprintln!("[leak] Detected {} non-finalized plugin boxes:", m.len());
+        for ((ty, id), _) in m.iter() {
+            eprintln!("  - {}(id={}) not finalized (missing fini or scope)", ty, id);
+        }
+    }
+}
+
+static REPORTER: Lazy<Reporter> = Lazy::new(|| Reporter);
+