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.cpp

@@ -1,8 +1,12 @@
-
-
 #include "Subject.h"
 #include "Observer.h"
 
-// TODO: Implement the Subject::attach() member function that adds an Observer to the set
+void Subject::attach(Observer* observer) {
+    m_observers.insert(observer);
+}
 
-// TODO: Implement the Subject::notify() member function that calls all the update functions
+void Subject::notify() {
+    for (auto* observer : m_observers) {
+        observer->update();
+    }
+}