perf(async): optimize coroutine fast-path and remove redundant error helper

- introduce post_handle() fast path across scheduler/io_context
- remove lambda-based coroutine resumption (direct handle posting)
- optimize asio bridge (awaitable, tcp, udp, dns)
- align thread_pool with coroutine fast-path (ctx_post_handle)
- optimize timer and signal wakeups using handle-based resume
- refactor when_all/when_any to use scheduler fast path
- cleanup spawn_detached to avoid lambda overhead
- remove duplicate cancelled_ec() from error.hpp (fix redefinition)
- overall reduction of allocations and indirections in async hot path

Author: GaspardKirira

include/vix/async/core/error.hpp

@@ -19,6 +19,7 @@
 #include <cstdint>
 #include <string>
 #include <system_error>
+#include <type_traits>
 
 namespace vix::async::core
 {
@@ -114,7 +115,7 @@ namespace vix::async::core
      *
      * @return Category name ("async").
      */
-    const char *name() const noexcept override
+    [[nodiscard]] const char *name() const noexcept override
     {
       return "async";
     }
@@ -125,7 +126,7 @@ namespace vix::async::core
      * @param c Integer value of the error code.
      * @return Human-readable error message.
      */
-    std::string message(int c) const override
+    [[nodiscard]] std::string message(int c) const override
     {
       switch (static_cast<errc>(c))
       {
@@ -162,7 +163,7 @@ namespace vix::async::core
    *
    * @return Reference to the async error category.
    */
-  inline const std::error_category &category() noexcept
+  [[nodiscard]] inline const std::error_category &category() noexcept
   {
     static error_category cat;
     return cat;
@@ -174,7 +175,7 @@ namespace vix::async::core
    * @param e Async error code.
    * @return Corresponding std::error_code.
    */
-  inline std::error_code make_error_code(errc e) noexcept
+  [[nodiscard]] inline std::error_code make_error_code(errc e) noexcept
   {
     return {static_cast<int>(e), category()};
   }

include/vix/async/core/io_context.hpp

@@ -100,7 +100,9 @@ namespace vix::async::core
     }
 
     /**
-     * @brief Post a callable task to the scheduler.
+     * @brief Post a generic callable task to the scheduler.
+     *
+     * This uses the scheduler generic path and is intended for ordinary callbacks.
      *
      * @tparam Fn Callable type.
      * @param fn Task to execute.
@@ -112,13 +114,23 @@ namespace vix::async::core
     }
 
     /**
-     * @brief Post a coroutine handle to the scheduler.
+     * @brief Post a coroutine handle to the scheduler fast path.
+     *
+     * @param h Coroutine handle to resume.
+     */
+    void post(std::coroutine_handle<> h) noexcept
+    {
+      sched_.post_handle(h);
+    }
+
+    /**
+     * @brief Explicit fast-path API for coroutine resumption.
      *
      * @param h Coroutine handle to resume.
      */
-    void post(std::coroutine_handle<> h)
+    void post_handle(std::coroutine_handle<> h) noexcept
     {
-      sched_.post(h);
+      sched_.post_handle(h);
     }
 
     /**
@@ -134,7 +146,7 @@ namespace vix::async::core
     /**
      * @brief Stop the scheduler.
      *
-     * Causes run() to exit.
+     * Causes run() to exit after draining pending work.
      */
     void stop() noexcept
     {