show threat budget in debug text overlay

src/lib/sim/WaveSystem.h

@@ -46,6 +46,10 @@ public:
     // Ticks remaining until the next boss wave fires (REQ-WAV-BOSS-COUNTDOWN).
     Tick bossCountdownTicks() const;
 
+    // Ticks remaining on the current normal-wave gap timer (REQ-WAV-GAP).
+    // Frozen during quiet windows.
+    Tick normalGapRemainingTicks() const;
+
 private:
     struct SpawnEntry
     {