feat(cmat): implement Ex41 observer pattern

Refactor the Pelican crossing exercise to use the Observer
pattern: Pelican acts as Subject and each Pole observes it,
replacing direct calls through the owned Pole pointer.

Co-authored-by: Qwen 3.8 27B <qwen3.8_27b-q3_k_xl@lmstudio.local>

cmat/Ex41/Subject.h

@@ -8,11 +8,11 @@ class Observer;
 
 class Subject {
   public:
-    // TODO: Declare the attach() function that adds an Observer to the set
+    void attach(Observer* observer);
   protected:
-    // TODO: Declare the notify() function that calls all the update functions
+    void notify();
   private:
-    // TODO: Define a set of Observer pointers
+    std::set<Observer*> m_observers;
 };
 
 #endif