refactor(Ex22): apply template method to Timer

Rename the virtual tick() override in PelicanTimer to doExpired()
and give the base Timer class a non-virtual tick() that checks for
expiration and dispatches to doExpired(). The exercise starting
point now matches SolutionEx22.

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

cmat/Ex22/Timer.h

@@ -1,27 +1,30 @@
-// ======================================================
-//
-// A general-purpose countdown timer
-// To use this class, specialize it
-// and override tick()
-//
-// ======================================================
-
-#ifndef timer_H
-#define timer_H
-
-class Timer {
-  public:
-    Timer();
-
-    void timeoutRequest(int seconds);
-
-    virtual void tick() = 0;
-
-  protected:
-    bool isExpired();
-
-  private:
-    int timeToGo;
-};
-
-#endif
+// ======================================================
+//
+// A general-purpose countdown timer
+// To use this class, specialize it
+// and override tick()
+//
+// ======================================================
+
+#ifndef timer_H
+#define timer_H
+
+class Timer {
+  public:
+    Timer();
+
+    void timeoutRequest(int seconds);
+
+   void tick();
+
+  private:
+    virtual void doExpired() = 0;
+
+  protected:
+    bool isExpired();
+
+  private:
+    int timeToGo;
+};
+
+#endif