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/Observer.h

@@ -1,10 +1,12 @@
-
-#ifndef OBSERVER_H
-#define OBSERVER_H
-
-class Observer {
-  public:
-    virtual void update() = 0;
-};
-
-#endif
+
+#ifndef OBSERVER_H
+#define OBSERVER_H
+
+class Hint;
+
+class Observer {
+  public:
+    virtual void update(const Hint&) = 0;
+};
+
+#endif