feat(cmat): pass hints to Ex42 observers

Extend the Ex42 observer pattern with Hint objects so each
observer receives the reason for the update instead of
querying the subject:

- Subject::notify() and Observer::update() now take a
  const Hint& argument.
- Add LightHint and WaitHint carrying the traffic light
  state and WAIT button state, defined in Pelican.h.
- Pole dispatches on hint type using dynamic_cast.
- The Pole constructor sets its initial lights directly
  from the Pelican's current state, since no notification
  is sent at startup. Stopgap until the initial update
  is handled properly.

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

cmat/Ex42/Subject.h

@@ -1,20 +1,25 @@
-
-
-#ifndef SUBJECT_H
-#define SUBJECT_H
-
-class Observer;
-
-#include <set>
-
-class Subject {
-  public:
-    typedef std::set<Observer *> ObsList;
-    void attach(Observer &obs);
-    void notify() const;
-
-  private:
-    ObsList myObservers;
-};
-
-#endif
+
+
+#ifndef SUBJECT_H
+#define SUBJECT_H
+
+class Observer;
+
+#include <set>
+
+class Hint {
+  public:
+    virtual ~Hint() = default;
+};
+
+class Subject {
+  public:
+    typedef std::set<Observer *> ObsList;
+    void attach(Observer &obs);
+    void notify(const Hint&) const;
+
+  private:
+    ObsList myObservers;
+};
+
+#endif