Async: add sh_ptr<ContinuationChainLink> to Callback<>

This change enables us to finally implement the tracing of
continuations backward from the point of acquisition for deadlock
debugging.

include/callback.h

@@ -7,7 +7,7 @@
 namespace smo {
 
 // Forward declaration
-class AsyncContinuation;
+class AsynchronousContinuationChainLink;
 
 /**
  * @brief Callback class that wraps a function and its caller continuation
@@ -15,23 +15,16 @@ class AsyncContinuation;
  * This class provides a way to pass both a callback function and the
  * caller's continuation in a single object, enabling deadlock detection
  * by walking the chain of continuations.
+ *
+ * Usage: Callback<CbFnT>{context, std::bind(...)}
  */
 template<typename CbFnT>
 class Callback
 {
 public:
-	/**
-	 * @brief Constructor
-	 * @param caller The caller's continuation
-	 * @param cb The callback function to invoke
-	 */
-	Callback(std::shared_ptr<AsyncContinuation> caller, std::function<CbFnT> cb)
-	: callerContinuation(std::move(caller)), callback(std::move(cb))
-	{}
-
-public:
-	std::shared_ptr<AsyncContinuation> callerContinuation;
-	std::function<CbFnT> callback;
+	// Aggregate initialization allows: Callback<CbFnT>{context, std::bind(...)}
+	std::shared_ptr<AsynchronousContinuationChainLink> callerContinuation;
+	CbFnT callbackFn;
 };
 
 } // namespace smo