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/Observer.h

@@ -4,7 +4,7 @@
 
 class Observer {
   public:
-    // TODO: declare the pure virtual function update()
+      virtual void update() = 0;
 };
 
 #endif