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/core.rs

@@ -228,6 +228,9 @@ pub struct NyashInterpreter {
     /// 共有ランタイム（Boxレジストリ等）
     #[allow(dead_code)]
     pub(super) runtime: NyashRuntime,
+
+    /// 現在の文脈で式結果が破棄されるか（must_use警告用）
+    pub(super) discard_context: bool,
 }
 
 impl NyashInterpreter {
@@ -259,6 +262,7 @@ impl NyashInterpreter {
             invalidated_ids: Arc::new(Mutex::new(HashSet::new())),
             stdlib: None, // 遅延初期化
             runtime,
+            discard_context: false,
         }
     }
 
@@ -292,6 +296,7 @@ impl NyashInterpreter {
             invalidated_ids: Arc::new(Mutex::new(HashSet::new())),
             stdlib: None,
             runtime,
+            discard_context: false,
         }
     }
     
@@ -321,6 +326,7 @@ impl NyashInterpreter {
             invalidated_ids: Arc::new(Mutex::new(HashSet::new())),
             stdlib: None, // 遅延初期化
             runtime,
+            discard_context: false,
         }
     }
 
@@ -351,6 +357,7 @@ impl NyashInterpreter {
             invalidated_ids: Arc::new(Mutex::new(HashSet::new())),
             stdlib: None,
             runtime,
+            discard_context: false,
         }
     }
     
@@ -379,8 +386,12 @@ impl NyashInterpreter {
             ASTNode::Program { statements, .. } => {
                 let mut result: Box<dyn NyashBox> = Box::new(VoidBox::new());
                 
-                for statement in statements.iter() {
+                let last = statements.len().saturating_sub(1);
+                for (i, statement) in statements.iter().enumerate() {
+                    let prev = self.discard_context;
+                    self.discard_context = i != last; // 最終文以外は値が破棄される
                     result = self.execute_statement(statement)?;
+                    self.discard_context = prev;
                     
                     // 制御フローチェック
                     match &self.control_flow {

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 {

src/interpreter/statements.rs

@@ -20,6 +20,26 @@ macro_rules! debug_trace {
 }
 
 impl NyashInterpreter {
+    fn warn_if_must_use(&self, value: &Box<dyn NyashBox>) {
+        if std::env::var("NYASH_LINT_MUSTUSE").unwrap_or_default() != "1" { return; }
+        if !self.discard_context { return; }
+        // 重資源のヒューリスティクス: プラグインBox、またはHTTP/Socket/File系の型名
+        #[cfg(all(feature = "plugins", not(target_arch = "wasm32")))]
+        {
+            if value.as_any().downcast_ref::<crate::runtime::plugin_loader_v2::PluginBoxV2>().is_some() {
+                eprintln!("[lint:must_use] Discarded resource value (plugin box). Consider assigning it or calling fini().");
+                return;
+            }
+        }
+        let ty = value.type_name();
+        let heavy = matches!(ty,
+            "FileBox" | "SocketBox" | "SocketServerBox" | "SocketClientBox" | "SocketConnBox" |
+            "HTTPServerBox" | "HTTPRequestBox" | "HTTPResponseBox" | "HttpClientBox"
+        );
+        if heavy {
+            eprintln!("[lint:must_use] Discarded {} value. Consider assigning it or calling fini().", ty);
+        }
+    }
     /// 文を実行 - Core statement execution engine
     pub(super) fn execute_statement(&mut self, statement: &ASTNode) -> Result<Box<dyn NyashBox>, RuntimeError> {
         match statement {
@@ -180,8 +200,12 @@ impl NyashInterpreter {
                 Ok(Box::new(VoidBox::new()))
             }
             
-            // 式文
-            _ => self.execute_expression(statement),
+            // 式文（結果は多くの場合破棄されるため、must_use警告を出力）
+            _ => {
+                let v = self.execute_expression(statement)?;
+                self.warn_if_must_use(&v);
+                Ok(v)
+            },
         }
     }