add boss wave counter and countdown to the title bar

src/lib/sim/WaveSystem.h

@@ -40,6 +40,12 @@ public:
     // incremented by 1 after each push — REQ-PSH-STATION-STATS).
     int generation() const;
 
+    // Boss wave counter (REQ-WAV-BOSS-COUNTER): current cycle number, starts at 1.
+    int bossWaveCounter() const;
+
+    // Ticks remaining until the next boss wave fires (REQ-WAV-BOSS-COUNTDOWN).
+    Tick bossCountdownTicks() const;
+
 private:
     struct SpawnEntry
     {